[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.*}

ArrowBuf の getActualConsumedMemory() と getPossibleConsumedMemory() から始め、空のバッファーの扱いを他のメモリ accounting メソッドと比較してください。空のバッファーで期待される動作を確認し、関連する ArrowBuf テストで両方のメソッドを検証してください。

索引モデルが issue の本文から書いたものです。