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ARROW-8386: [Python] Fix error when pyarrow.jvm gets an empty vector

When `pyarrow.jvm` gets an empty Vector from the JVM, the buffer list returned is empty and then fails with a `ValueError` in `pa.Array.from_buffers` because it still expects a list populated with buffers. This change checks if the JVM vector has a value count of 0, then manually creates an empty pyarrow Array of the same type.

Closes #6889 from BryanCutler/python-jvm-empty-array-ARROW-8386

Authored-by: Bryan Cutler <cutlerb@gmail.com>
Signed-off-by: Bryan Cutler <cutlerb@gmail.com>