Fixing empty input to getMapValue crashing (#9262)

This changes the calls in java/cudf to check for an empty input and return an empty result instead of crashing.

Fixes #9253

Authors:
  - Mike Wilson (https://github.com/hyperbolic2346)

Approvers:
  - Jason Lowe (https://github.com/jlowe)

URL: #9262

java/src/main/native/src/map_lookup.cu

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, NVIDIA CORPORATION.
+ * Copyright (c) 2020-2021, NVIDIA CORPORATION.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -183,6 +183,10 @@ std::unique_ptr<column> map_lookup(column_view const &map_column, string_scalar
   // Defensive checks.
   map_input_check(map_column, stream);
 
+  if (map_column.size() == 0) {
+    return make_empty_column(cudf::data_type{cudf::type_id::STRING});
+  }
+
   lists_column_view lcv{map_column};
   column_view structs_column = lcv.get_sliced_child(stream);
   // Two-pass plan: construct gather map, and then gather() on structs_column.child(1). Plan A.

java/src/test/java/ai/rapids/cudf/ColumnVectorTest.java

@@ -5412,6 +5412,17 @@ void testGetMapValue() {
     }
   }
 
+  @Test
+  void testGetMapValueEmptyInput() {
+    HostColumnVector.StructType structType = new HostColumnVector.StructType(true, Arrays.asList(new HostColumnVector.BasicType(true, DType.STRING),
+        new HostColumnVector.BasicType(true, DType.STRING)));
+    try (ColumnVector cv = ColumnVector.fromLists(new HostColumnVector.ListType(true, structType));
+         ColumnVector res = cv.getMapValue(Scalar.fromString("a"));
+         ColumnVector expected = ColumnVector.fromStrings()) {
+      assertColumnsAreEqual(expected, res);
+    }
+  }
+
   @Test
   void testGetMapKeyExistence() {
     List<HostColumnVector.StructData> list1 = Arrays.asList(new HostColumnVector.StructData("a", "b"));