apachegh-13340: Allow adding a parent field to an index with no fields

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

@@ -1122,6 +1122,7 @@ public IndexWriter(Directory d, IndexWriterConfig conf) throws IOException {
       globalFieldNumberMap = getFieldNumberMap();
       if (create == false
           && conf.getParentField() != null
+          && globalFieldNumberMap.getFieldNames().isEmpty() == false
           && globalFieldNumberMap.getFieldNames().contains(conf.getParentField()) == false) {
         throw new IllegalArgumentException(
             "can't add a parent field to an already existing index without a parent field");

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

@@ -4851,7 +4851,9 @@ public void testParentFieldExistingIndex() throws IOException {
     try (Directory dir = newDirectory()) {
       IndexWriterConfig iwc = new IndexWriterConfig(new MockAnalyzer(random()));
       try (IndexWriter writer = new IndexWriter(dir, iwc)) {
-        writer.addDocument(new Document());
+        Document d = new Document();
+        d.add(new TextField("f", "a", Field.Store.NO));
+        writer.addDocument(d);
       }
       IllegalArgumentException iae =
           expectThrows(
@@ -4959,4 +4961,19 @@ public void testParentFieldIsAlreadyUsed() throws IOException {
           iae.getMessage());
     }
   }
+
+  public void testParentFieldEmptyIndex() throws IOException {
+    try (Directory dir = newMockDirectory()) {
+      IndexWriterConfig iwc = new IndexWriterConfig(new MockAnalyzer(random()));
+      iwc.setParentField("parent");
+      try (IndexWriter writer = new IndexWriter(dir, iwc)) {
+        writer.commit();
+      }
+      IndexWriterConfig iwc2 = new IndexWriterConfig(new MockAnalyzer(random()));
+      iwc2.setParentField("parent");
+      try (IndexWriter writer = new IndexWriter(dir, iwc2)) {
+        writer.commit();
+      }
+    }
+  }
 }