Make typing checks more strict (#14429)

## Description:

Make typing checks more strict: add `--strict-optional` flag that forbids implicit None return type. This flag will become default in the next version of mypy (0.600)

Add `homeassistant/util/` to checked dirs.

## Checklist:
  - [x] The code change is tested and works locally.
  - [x] Local tests pass with `tox`. **Your PR cannot be merged unless tests pass**

homeassistant/util/json.py

@@ -8,8 +8,6 @@ from homeassistant.exceptions import HomeAssistantError
 
 _LOGGER = logging.getLogger(__name__)
 
-_UNDEFINED = object()
-
 
 class SerializationError(HomeAssistantError):
     """Error serializing the data to JSON."""
@@ -19,7 +17,7 @@ class WriteError(HomeAssistantError):
     """Error writing the data."""
 
 
-def load_json(filename: str, default: Union[List, Dict] = _UNDEFINED) \
+def load_json(filename: str, default: Union[List, Dict, None] = None) \
         -> Union[List, Dict]:
     """Load JSON data from a file and return as dict or list.
 
@@ -37,7 +35,7 @@ def load_json(filename: str, default: Union[List, Dict] = _UNDEFINED) \
     except OSError as error:
         _LOGGER.exception('JSON file reading failed: %s', filename)
         raise HomeAssistantError(error)
-    return {} if default is _UNDEFINED else default
+    return {} if default is None else default
 
 
 def save_json(filename: str, data: Union[List, Dict]):
@@ -46,9 +44,9 @@ def save_json(filename: str, data: Union[List, Dict]):
     Returns True on success.
     """
     try:
-        data = json.dumps(data, sort_keys=True, indent=4)
+        json_data = json.dumps(data, sort_keys=True, indent=4)
         with open(filename, 'w', encoding='utf-8') as fdesc:
-            fdesc.write(data)
+            fdesc.write(json_data)
     except TypeError as error:
         _LOGGER.exception('Failed to serialize to JSON: %s',
                           filename)