[FIX] runbot_merge: v13 computed field compatibility

In v13, computed fields *must* have their value set on all records.

runbot_merge/models/pull_requests.py

@@ -604,6 +604,7 @@ class PullRequests(models.Model):
         for s in self:
             c = Commits.search([('sha', '=', s.head)])
             if not (c and c.statuses):
+                s.status = s.statuses = False
                 continue
 
             statuses = json.loads(c.statuses)