Add changelog, change code to guard clause

CHANGELOG

@@ -35,6 +35,9 @@ v 8.7.0 (unreleased)
   - ClosingIssueExtractor regex now also works with colons. e.g. "Fixes: #1234" !3591
   - Update number of Todos in the sidebar when it's marked as "Done". !3600
 
+v 8.6.6
+  - Fix error on language detection when repository has no HEAD (e.g., master branch). !3654
+
 v 8.6.5
   - Fix importing from GitHub Enterprise. !3529
   - Perform the language detection after updating merge requests in `GitPushService`, leading to faster visual feedback for the end-user. !3533

app/models/repository.rb

@@ -896,9 +896,9 @@ class Repository
   end
 
   def main_language
-    unless empty? or rugged.head_unborn?
-      Linguist::Repository.new(rugged, rugged.head.target_id).language
-    end
+    return nil if empty? || rugged.head_unborn?
+
+    Linguist::Repository.new(rugged, rugged.head.target_id).language
   end
 
   def avatar