Fix a bug in the `Configuration provider to correctly handle overriding by None`.

src/dependency_injector/providers.pyx

@@ -1507,6 +1507,8 @@ cdef class Configuration(Object):
         :return: Overriding context.
         :rtype: :py:class:`OverridingContext`
         """
+        if provider is None:
+            provider = {}
         context = super().override(provider)
         self.reset_cache()
         return context

tests/unit/providers/test_configuration_py2_py3.py

@@ -185,6 +185,13 @@ class ConfigTests(unittest.TestCase):
         with self.assertRaises(AttributeError):
             a.__name__
 
+    def test_missing_key(self):
+        # See: https://github.com/ets-labs/python-dependency-injector/issues/358
+        self.config.override(None)
+        value = self.config.key()
+
+        self.assertIsNone(value)
+
     def test_deepcopy(self):
         provider = providers.Configuration('config')
         provider_copy = providers.deepcopy(provider)