Cover the case where the action.output is None

Author: cderici

juju/action.py

@@ -14,7 +14,8 @@ def status(self):
 
     async def fetch_output(self):
         completed_action = await self.model._get_completed_action(self.id)
-        self.results = completed_action.output
+        self.results = {} if completed_action.output is None else \
+            completed_action.output
         self._status = completed_action.status
 
     async def wait(self):