fix stuck mr

fixes #348. related to 64874193.

CHANGELOG

@@ -73,6 +73,7 @@ v 7.10.0 (unreleased)
   - Fix git over ssh errors 'fatal: protocol error: bad line length character' 
   - Automatically setup GitLab CI project for forks if origin project has GitLab CI enabled
   - Bust group page project list cache when namespace name or path changes.
+  - Fix stuck Merge Request merging events from old installations (Ben Bodenmiller)
 
 v 7.9.3
   - Contains no changes

app/models/merge_request.rb

@@ -361,6 +361,8 @@ def source_branches
   end
 
   def locked_long_ago?
-    locked_at && locked_at < (Time.now - 1.day)
+    return false unless locked?
+
+    locked_at.nil? || locked_at < (Time.now - 1.day)
   end
 end