Don't instantinate a Model if attributes are empty in Foreign relation

Adds guard for scenarios where the `Foreign` relationship is passed an
empty set of properties for the target class.

pyresto/core.py

@@ -405,8 +405,8 @@ def __get__(self, instance, owner):
 
         if instance not in self.__cache:
             if self.__embedded:
-                self.__cache[instance] = self.__model(
-                    **getattr(instance, self.__key_property))
+                properties = getattr(instance, self.__key_property)
+                self.__cache[instance] = self.__model(**properties) if properties else None
                 self.__cache[instance]._auth = instance._auth
             else:
                 self.__cache[instance] = self.__model.get(