Prevent PointValues from returning null for ghost fields

getPointValues may currently return null for unknown fields or fields that don't index points.
It can happen that a field no longer has points for any document in a segment after delete+merge, which
causes field info to think that the field is there and has points, yet when calling getPointValues null
is returned.

With this change, we prevent getPointValues from returning null for ghost fields, it will instead return an
empty instance of PointValues.

Relates to apache#11393

lucene/core/src/java/org/apache/lucene/codecs/lucene90/Lucene90PointsReader.java

@@ -137,7 +137,8 @@ public PointValues getValues(String fieldName) {
       throw new IllegalArgumentException("field=\"" + fieldName + "\" did not index point values");
     }
 
-    return readers.get(fieldInfo.number);
+    PointValues pointValues = readers.get(fieldInfo.number);
+    return pointValues == null ? PointValues.EMPTY : pointValues;
   }
 
   @Override

lucene/core/src/java/org/apache/lucene/index/PointValues.java

@@ -467,4 +467,91 @@ public final long estimateDocCount(IntersectVisitor visitor) {
 
   /** Returns the total number of documents that have indexed at least one point. */
   public abstract int getDocCount();
+
+  private static final PointTree EMPTY_POINT_TREE =
+      new PointTree() {
+        @Override
+        public PointTree clone() {
+          throw new UnsupportedOperationException();
+        }
+
+        @Override
+        public boolean moveToChild() {
+          return false;
+        }
+
+        @Override
+        public boolean moveToSibling() {
+          return false;
+        }
+
+        @Override
+        public boolean moveToParent() {
+          return false;
+        }
+
+        @Override
+        public byte[] getMinPackedValue() {
+          return new byte[0];
+        }
+
+        @Override
+        public byte[] getMaxPackedValue() {
+          return new byte[0];
+        }
+
+        @Override
+        public long size() {
+          return 0;
+        }
+
+        @Override
+        public void visitDocIDs(IntersectVisitor visitor) {}
+
+        @Override
+        public void visitDocValues(IntersectVisitor visitor) {}
+      };
+
+  public static final PointValues EMPTY =
+      new PointValues() {
+        @Override
+        public PointTree getPointTree() {
+          return EMPTY_POINT_TREE;
+        }
+
+        @Override
+        public byte[] getMinPackedValue() {
+          return null;
+        }
+
+        @Override
+        public byte[] getMaxPackedValue() {
+          return null;
+        }
+
+        @Override
+        public int getNumDimensions() {
+          return 0;
+        }
+
+        @Override
+        public int getNumIndexDimensions() {
+          return 0;
+        }
+
+        @Override
+        public int getBytesPerDimension() {
+          return 0;
+        }
+
+        @Override
+        public long size() {
+          return 0;
+        }
+
+        @Override
+        public int getDocCount() {
+          return 0;
+        }
+      };
 }

lucene/core/src/test/org/apache/lucene/index/TestPointValues.java

@@ -646,7 +646,16 @@ public void testDeleteAllPointDocs() throws Exception {
 
     w.forceMerge(1);
     DirectoryReader r = DirectoryReader.open(w);
-    assertNull(r.leaves().get(0).reader().getPointValues("int"));
+    PointValues emptyValues = r.leaves().get(0).reader().getPointValues("int");
+    assertEquals(0, emptyValues.getDocCount());
+    assertEquals(0, emptyValues.size());
+    assertEquals(0, emptyValues.getNumDimensions());
+    assertEquals(0, emptyValues.getNumIndexDimensions());
+    assertEquals(0, emptyValues.getBytesPerDimension());
+    assertEquals(0, emptyValues.getPointTree().size());
+    assertFalse(emptyValues.getPointTree().moveToChild());
+    assertFalse(emptyValues.getPointTree().moveToParent());
+    assertFalse(emptyValues.getPointTree().moveToSibling());
     w.close();
     r.close();
     dir.close();

lucene/core/src/test/org/apache/lucene/search/TestFieldExistsQuery.java

@@ -702,6 +702,28 @@ private float[] randomVector(int dim) {
     return v;
   }
 
+  public void testDeleteAllPointDocs() throws Exception {
+    try (Directory dir = newDirectory();
+        RandomIndexWriter iw = new RandomIndexWriter(random(), dir)) {
+
+      Document doc = new Document();
+      doc.add(new StringField("id", "0", Field.Store.NO));
+      doc.add(new LongPoint("long", 17));
+      doc.add(new NumericDocValuesField("long", 17));
+      iw.addDocument(doc);
+      iw.addDocument(new Document());
+      iw.commit();
+
+      iw.deleteDocuments(new Term("id", "0"));
+      iw.forceMerge(1);
+
+      try (IndexReader reader = iw.getReader()) {
+        IndexSearcher searcher = newSearcher(reader);
+        assertEquals(0, searcher.count(new FieldExistsQuery("long")));
+      }
+    }
+  }
+
   private void assertSameMatches(IndexSearcher searcher, Query q1, Query q2, boolean scores)
       throws IOException {
     final int maxDoc = searcher.getIndexReader().maxDoc();

lucene/test-framework/src/java/org/apache/lucene/tests/index/AssertingLeafReader.java

@@ -20,7 +20,6 @@
 import java.util.Arrays;
 import java.util.Iterator;
 import java.util.List;
-import java.util.Objects;
 import org.apache.lucene.index.BinaryDocValues;
 import org.apache.lucene.index.DocValues;
 import org.apache.lucene.index.DocValuesType;
@@ -1112,8 +1111,8 @@ public PointValues getWrapped() {
     }
 
     private void assertStats(int maxDoc) {
-      assert in.size() > 0;
-      assert in.getDocCount() > 0;
+      assert in.size() >= 0;
+      assert in.getDocCount() >= 0;
       assert in.getDocCount() <= in.size();
       assert in.getDocCount() <= maxDoc;
     }
@@ -1127,13 +1126,13 @@ public PointTree getPointTree() throws IOException {
     @Override
     public byte[] getMinPackedValue() throws IOException {
       assertThread("Points", creationThread);
-      return Objects.requireNonNull(in.getMinPackedValue());
+      return in.getMinPackedValue();
     }
 
     @Override
     public byte[] getMaxPackedValue() throws IOException {
       assertThread("Points", creationThread);
-      return Objects.requireNonNull(in.getMaxPackedValue());
+      return in.getMaxPackedValue();
     }
 
     @Override
@@ -1210,7 +1209,7 @@ public byte[] getMaxPackedValue() {
     @Override
     public long size() {
       final long size = in.size();
-      assert size > 0;
+      assert size >= 0;
       return size;
     }