pybind: fix tests that do not fail as expected

pybind: fix tests that do not fail as expected

A missing argument make the test fail indeed, but the intended test is
to demonstrate something else ( either character validation or excess of
arguments etc. ). The result is {} instead of None which is what should
have been expected in the first place.

Ideally there would be a more verbose way to check for syntactic errors
to make such mistakes less probable.

Reviewed-By: Christophe Courtaut <christophe.courtaut@gmail.com>
Signed-off-by: Loic Dachary <loic@dachary.org>