Do not try to fill None

Use case: videoob play on an unknown URL.
The get_object(... , ['url']) would throw a FieldNotFound, when the
right error should be video not found.

weboob/tools/backend.py

@@ -354,6 +354,8 @@ class BaseBackend(object):
         :param fields: what fields to fill; if None, all fields are filled
         :type fields: :class:`list`
         """
+        if obj is None:
+            return obj
 
         def not_loaded(v):
             return (v is NotLoaded or isinstance(v, CapBaseObject) and not v.__iscomplete__())