[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.*}

Comece em ArrowBuf, em getActualConsumedMemory() e getPossibleConsumedMemory(), comparando o tratamento de buffers vazios com o dos outros métodos de contabilização de memória. Confirme o comportamento esperado para o buffer vazio e verifique ambos os métodos com os testes relevantes de ArrowBuf.

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