python: cleanup the __nonzero__ implementations

python: cleanup the __nonzero__ implementations

Cleanup the code, reword the docstring and use the same implementation
in the Threads, Tags and Messages classes.

__nonzero__ implements truth value testing. If __nonzero__ is not
implemented, the python runtime would fall back to `len(..) > 0` thus
exhausting the iterator.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>