[Java] offset buffer for vector of variable-width type with zero value count is empty

I am reporting what I think might be two related bugs in `org.apache.arrow.vector.BaseVariableWidthVector`
1. The offset buffer is initialized as empty. I expect that it to have 4 bytes that represent the integer zero.
1. The `getBufferSize` method returns 0 when value count is zero, instead of 4.

Compare to the pyarrow implementation, which I believe correctly populates the offset buffer:
```java

>>> import pyarrow as pa
>>> array = pa.array([], type=pa.binary())
>>> array
[]
>>> print([b.hex().decode() for b in array.buffers()])
['', '00000000', '']

```
 

**Reporter**: [Steve M. Kim](https://issues.apache.org/jira/browse/ARROW-7342)

_{**Note**: *This issue was originally created as [ARROW-7342](https://issues.apache.org/jira/browse/ARROW-7342). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

从 org.apache.arrow.vector.BaseVariableWidthVector 开始，重点关注值计数为零时的偏移缓冲区初始化和 getBufferSize。将 Java 的行为与报告中展示的空 pyarrow 二进制数组进行比较；当零值情况下暴露出预期的四字节零偏移量并报告相应的缓冲区大小时，即完成。

由索引模型根据 Issue 内容生成。