add guard clause to MergeRequest#issues_mentioned_but_not_closing

app/models/merge_request.rb

@@ -569,17 +569,14 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def issues_mentioned_but_not_closing(current_user = self.author)
-    issues = []
-    closing_issues = []
+    return [] unless target_branch == project.default_branch
 
-    if target_branch == project.default_branch
     ext = Gitlab::ReferenceExtractor.new(project, current_user)
     ext.analyze(description)
 
     issues = ext.issues
     closing_issues = Gitlab::ClosingIssueExtractor.new(project, current_user).
       closed_by_message(description)
-    end
 
     issues - closing_issues
   end