Make the subject URL optional

This goes against the documentation at
https://docs.github.com/en/rest/activity/notifications but it seems to be
required in order to handle certain types of notifications.

src/GitHub/Data/Activities.hs

@@ -26,7 +26,7 @@ instance FromJSON RepoStarred where
 
 data Subject = Subject
     { subjectTitle :: !Text
-    , subjectURL :: !URL
+    , subjectURL :: !(Maybe URL)
     , subjectLatestCommentURL :: !(Maybe URL)
     -- https://developer.github.com/v3/activity/notifications/ doesn't indicate
     -- what the possible values for this field are.