VectorAppender fails with OversizedAllocationException when appending to empty UnionVector

VectorAppender.java の556行目付近のループから始め、NonNullableStructVector.java の373行目付近も含めて UnionVector.getValueCapacity() を調査します。提供されている testAppendEmptyTargetUnionVector の再現を実行し、reAlloc() の前後で容量を比較します。空の UnionVector への追加が OversizedAllocationException なしで完了し、5つすべての float 値が保持されれば完了です。

Describe the bug, including details regarding any error messages, version, and platform.

Description:
When appending a UnionVector to an empty target UnionVector, this while loop runs infinitely until the exception occurs:

org.apache.arrow.vector.util.OversizedAllocationException: Unable to expand the buffer

It seems to be a issue with the UnionVector.getValueCapacity() method. Logging the capacity before and after calling reAlloc() shows that it's always 0.
Debugging further, it seems like the value capacity of the internalStruct in the UnionVector always returns 0 because it has no children so size is 0.

 @Override
  public int getValueCapacity() {
    return Math.min(getTypeBufferValueCapacity(), internalStruct.getValueCapacity());
  }

Steps to reproduce

  @Test
  public void testAppendEmptyTargetUnionVector() {
    final int length = 5;

    try (final UnionVector target = UnionVector.empty("target", allocator);
         final UnionVector delta = UnionVector.empty("delta", allocator)) {

      // populate the delta vector
      delta.setType(0, Types.MinorType.FLOAT4);
      delta.setType(1, Types.MinorType.FLOAT4);
      delta.setType(2, Types.MinorType.FLOAT4);
      delta.setType(3, Types.MinorType.FLOAT4);
      delta.setType(4, Types.MinorType.FLOAT4);

      Float4Vector deltaFloatVector = delta.getFloat4Vector();
      deltaFloatVector.allocateNew();
      ValueVectorDataPopulator.setVector(deltaFloatVector, 1f, 2f, 3f, 4f, 5f);
      assertEquals(length, deltaFloatVector.getValueCount());
      delta.setValueCount(length);

      VectorAppender appender = new VectorAppender(target);
      delta.accept(appender, null);

      assertEquals(length, target.getValueCount());

      for (int i = 0; i < length; i++) {
        Object floatObj = target.getObject(i);
        assertTrue(floatObj instanceof Float);
        assertEquals(i+1, ((Float) floatObj).intValue());
      }
    }
  }

This issue does not occur if any value is set in the target vector before appending.