Fix a reported bug when bandit encounters "__import__()"

Parsing "__import__()" results in an exception from the test
blacklist_import_func, since it assumes the the call will have a
parameter. Calling __import__ without a parameter is not valid
python, however this bug has been seen in the wild.

Change-Id: Ia9476f19fa0b571c71a7410152e95757543ec5ea
Closes-bug: 1396333