Handle a display name / avatar URL not included in a federation request. (#9023)

These may be omitted if not set, but Synapse assumed they would
be in the response.

changelog.d/9023.bugfix

@@ -0,0 +1 @@
+Fix a longstanding issue where an internal server error would occur when requesting a profile over federation that did not include a display name / avatar URL.

synapse/handlers/profile.py

@@ -156,7 +156,7 @@ class ProfileHandler(BaseHandler):
             except HttpResponseException as e:
                 raise e.to_synapse_error()
 
-            return result["displayname"]
+            return result.get("displayname")
 
     async def set_displayname(
         self,
@@ -246,7 +246,7 @@ class ProfileHandler(BaseHandler):
             except HttpResponseException as e:
                 raise e.to_synapse_error()
 
-            return result["avatar_url"]
+            return result.get("avatar_url")
 
     async def set_avatar_url(
         self,