[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.*}

Empieza en ArrowBuf, en getActualConsumedMemory() y getPossibleConsumedMemory(), comparando cómo manejan los buffers vacíos con los otros métodos de contabilidad de memoria. Confirma el comportamiento esperado para el buffer vacío y verifica ambos métodos con las pruebas relevantes de ArrowBuf.

Escrito por el modelo de indexación a partir del texto del issue.