[Java] Check if ArrowBuf is empty buffer in getActualConsumedMemory() and getPossibleConsumedMemory()

Most of the methods related to memory accounting in ArrowBuf have special handling for the case when then Buffer is the empty buffer instance. This check is missing in these two methods.

**Reporter**: [Steven Phillips](https://issues.apache.org/jira/browse/ARROW-791) / @StevenMPhillips

_{**Note**: *This issue was originally created as [ARROW-791](https://issues.apache.org/jira/browse/ARROW-791). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

从 ArrowBuf 中的 getActualConsumedMemory() 和 getPossibleConsumedMemory() 开始，将它们对空缓冲区的处理与其他内存核算方法进行比较。确认空缓冲区的预期行为，并使用相关的 ArrowBuf 测试验证这两个方法。

由索引模型根据 Issue 内容生成。