[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.*}

Start in ArrowBuf at getActualConsumedMemory() and getPossibleConsumedMemory(), comparing their empty-buffer handling with the other memory-accounting methods. Confirm the expected behavior for the empty buffer and verify both methods with the relevant ArrowBuf tests.

Written by the indexing model from the issue text.