VectorAppender fails with OversizedAllocationException when appending to empty UnionVector

Start with the VectorAppender.java loop around line 556 and inspect UnionVector.getValueCapacity(), including NonNullableStructVector.java around line 373. Run the provided testAppendEmptyTargetUnionVector reproduction and compare capacity before and after reAlloc(). Done means appending to an empty UnionVector completes without OversizedAllocationException and preserves all five float values.

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.