Check that the request payload data is valid json.

By adding `force=True` to request.get_json() the mime type is ignore. If the data is not valid json the method will return a `BadRequestError` we catch that and throw our own error with a clear error message "Invalid JSON supplied in POST data".
If the json is valid return the json data or an empty dict if None is passed in.

This PR improves the error messages if the json is invalid, previously, the error message was "None object type" message which is not very helpful.

tests/app/v2/notifications/test_post_notifications.py

@@ -931,4 +931,7 @@ def test_post_email_notification_when_data_is_empty_returns_400(
     )
     error_msg = json.loads(response.get_data(as_text=True))["errors"][0]["message"]
     assert response.status_code == 400
-    assert error_msg == 'Request body is empty.'
+    if notification_type == 'sms':
+        assert error_msg == 'phone_number is a required property'
+    else:
+        assert error_msg == 'email_address is a required property'