Merge branch 'feature/fix-14753' into 'master'

Fix #14753: Check if head is born before trying to detect main language

This MR makes sure that head exists before trying to detect the main language.
This prevents errors on repo's without a master branch.

Closes #14753

See merge request !3654

CHANGELOG

@@ -43,6 +43,9 @@ v 8.7.0 (unreleased)
   - API: Expose 'updated_at' for issue, snippet, and merge request notes (Robert Schilling)
   - API: User can leave a project through the API when not master or owner. !3613
 
+v 8.6.6
+  - Fix error on language detection when repository has no HEAD (e.g., master branch). !3654 (Jeroen Bobbeldijk)
+
 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?
-      Linguist::Repository.new(rugged, rugged.head.target_id).language
-    end
+    return if empty? || rugged.head_unborn?
+
+    Linguist::Repository.new(rugged, rugged.head.target_id).language
   end
 
   def avatar