[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.*}

從 ArrowBuf 中的 getActualConsumedMemory() 和 getPossibleConsumedMemory() 開始，將它們對空緩衝區的處理與其他記憶體核算方法進行比較。確認空緩衝區的預期行為，並使用相關的 ArrowBuf 測試驗證這兩個方法。

由索引模型根據 Issue 內容生成。