profile.yaml: require displayname/avatar_url

This makes a deliberate choice for the question stated in #2717.

data/api/client-server/profile.yaml

@@ -56,6 +56,7 @@ paths:
               displayname:
                 type: string
                 description: The new display name for this user.
+            required: ["displayname"]
       responses:
         200:
           description: The display name was set.
@@ -131,6 +132,7 @@ paths:
                 type: string
                 format: uri
                 description: The new avatar URL for this user.
+            required: ["avatar_url"]
       responses:
         200:
           description: The avatar URL was set.