[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.*}

Beginne in ArrowBuf bei getActualConsumedMemory() und getPossibleConsumedMemory() und vergleiche deren Behandlung leerer Buffer mit den anderen Methoden zur Speicherabrechnung. Bestätige das erwartete Verhalten für den leeren Buffer und überprüfe beide Methoden mit den relevanten ArrowBuf-Tests.

Vom Indexierungsmodell aus dem Issue-Text verfasst.