test: fix docstring validation function
test: fix docstring validation function

For some reason in Python even if using getattr with None and as
default value a string for the type __doc__ will return None.

>>> type(getattr(None, "__doc__", ""))
<class 'NoneType'>

The error reported was:

ft1.1: neutron.tests.functional.db.test_migrations.TestModelsMigrationsMySQL.test_branchestesttools.testresult.real._StringException: Traceback (most recent call last):
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/base.py", line 182, in func
    return f(self, *args, **kwargs)
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/functional/db/test_migrations.py", line 302, in test_branches
    find_migration_exceptions()
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/functional/db/test_migrations.py", line 253, in find_migration_exceptions
    if len(explanation) < 1:
TypeError: object of type 'NoneType' has no len()

Signed-off-by: Sahid Orentino Ferdjaoui <f799991784850b8d4b84e1fdbe797720df9aae1a@industrialdiscipline.com>
Change-Id: I7c03bac9c6e8dd074325f511e555c10fec8e77fc

openstack/neutron,openstack/neutron,mahak/neutron,mahak/neutron,mahak/neutron,openstack/neutron