arg_validator: accept None as valid input for ArgValidatorDict

Users might want to use jinja2 templates to set properties. As such,
it's convenient to accept None as an alias for an empty dictionary.

For exmaple, setting like `"match": None` will be allowed by the role:
e.g.
        network_connections:
          - name: enp0s8
            type: ethernet
            persistent_state: present
            state: up
            match:
            ip:
              route_metric4: 10

Signed-off-by: Wen Liang <liangwen12year@gmail.com>

module_utils/network_lsr/argument_validator.py

@@ -350,6 +350,11 @@ class ArgValidatorDict(ArgValidator):
     def _validate_impl(self, value, name):
         result = {}
         seen_keys = set()
+        if value is None:
+            # Users might want to use jinja2 templates to set properties. As such,
+            # it's convenient to accept None as an alias for an empty dictionary
+            # e.g. setting like `"match": None` will be allowed by the role
+            return {}
         try:
             items = list(value.items())
         except AttributeError:

tests/unit/test_network_connections.py

@@ -437,6 +437,7 @@ class TestValidator(unittest.TestCase):
             {"i": 5, "s": "s_default", "l": "6"}, v.validate({"i": "5", "l": "6"})
         )
         self.assertValidationError(v, {"k": 1})
+        self.assertEqual(v.validate(None), {})
 
     def test_validate_list(self):