Fix GET /_matrix/federation/v1/query/profile response (#18593)

Don't send the fields `avatar_url` and `displayname` when they are not
defined for the queried user.

Before this change they would be sent and set to null in the JSON
response object, which would violate the OpenAPI definitions
(https://spec.matrix.org/v1.11/server-server-api/#get_matrixfederationv1queryprofile).
Fixes: #18442 
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---------

Co-authored-by: Quentin Gliech <quenting@element.io>

changelog.d/18593.bugfix

@@ -0,0 +1 @@
+Fix `avatar_url` and `displayname` being sent on federation profile queries when they are not set.

synapse/handlers/profile.py

@@ -539,11 +539,17 @@ class ProfileHandler:
         response: JsonDict = {}
         try:
             if just_field is None or just_field == ProfileFields.DISPLAYNAME:
-                response["displayname"] = await self.store.get_profile_displayname(user)
-
+                displayname = await self.store.get_profile_displayname(user)
+                # do not set the displayname field if it is None,
+                # since then we send a null in the JSON response
+                if displayname is not None:
+                    response["displayname"] = displayname
             if just_field is None or just_field == ProfileFields.AVATAR_URL:
-                response["avatar_url"] = await self.store.get_profile_avatar_url(user)
-
+                avatar_url = await self.store.get_profile_avatar_url(user)
+                # do not set the avatar_url field if it is None,
+                # since then we send a null in the JSON response
+                if avatar_url is not None:
+                    response["avatar_url"] = avatar_url
             if self.hs.config.experimental.msc4133_enabled:
                 if just_field is None:
                     response.update(await self.store.get_profile_fields(user))