Issue #16828: Fix error incorrectly raised by bz2.compress('').

Patch by Martin Packman.
Issue #16828: Fix error incorrectly raised by bz2.compress('').

Initial patch by Martin Packman.
Correction: issue #16828 also affects BZ2Compressor.compress().
Issue #16828: Fix error incorrectly raised by bz2.compress(b'') and bz2.BZ2Compressor.compress(b'').

Initial patch by Martin Packman.