Add none test case to shell_args_list

As an improvement in I40e6f1c2ed71ab57d6597c086554186e233d7b94 we added
support to just return '' if the variable is None. This covers this case
with a test.

Change-Id: I61d7d4b104fb4dc895dc35b7a2460b92c6ab6e59

tests/plugins/filter/test_shell_args.py

@@ -37,3 +37,7 @@ class TestShellArgsFilters(tests_base.TestCase):
         expected = '--p a --p b'
         self.assertEqual(expected,
                          self.filter.shell_arg_list(arg, parameter='--p'))
+
+    def test_shell_arg_list_none(self):
+        expected = ''
+        self.assertEqual(expected, self.filter.shell_arg_list(None))