VectorAppender fails with OversizedAllocationException when appending to empty UnionVector

从 VectorAppender.java 第556行附近的循环开始，检查 UnionVector.getValueCapacity()，包括 NonNullableStructVector.java 第373行附近的代码。运行提供的 testAppendEmptyTargetUnionVector 复现，并比较 reAlloc() 前后的容量。当向空的 UnionVector 追加数据时能够在没有 OversizedAllocationException 的情况下完成，并保留全部五个 float 值，即表示完成。

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.