[FIX] runbot_merge: bugs in log of branch stagings (in frontend)

- Some batches in a few stagings are apparently empty (e.g. batch
  71771 from staging 32511), the listing was not resilient to such
  issues.

  Update the code to suppress the display of empty batches.
- The possibility of accessing `/runbot_merge/<id>` with a
  non-existent branch had not been considered and triggered a
  rendering error in the template.

Fixes #630, fixes #631

runbot_merge/controllers/dashboard.py

@@ -19,13 +19,17 @@ class MergebotDashboard(Controller):
 
     @route('/runbot_merge/<int:branch_id>', auth='public', type='http', website=True)
     def stagings(self, branch_id, until=None):
+        branch = request.env['runbot_merge.branch'].browse(branch_id).sudo().exists()
+        if not branch:
+            raise werkzeug.exceptions.NotFound()
+
         stagings = request.env['runbot_merge.stagings'].with_context(active_test=False).sudo().search([
-            ('target', '=', branch_id),
+            ('target', '=', branch.id),
             ('staged_at', '<=', until) if until else (True, '=', True),
         ], order='staged_at desc', limit=LIMIT+1)
 
         return request.render('runbot_merge.branch_stagings', {
-            'branch': request.env['runbot_merge.branch'].browse(branch_id).sudo(),
+            'branch': branch,
             'stagings': stagings[:LIMIT],
             'next': stagings[-1].staged_at if len(stagings) > LIMIT else None,
         })

runbot_merge/views/templates.xml

@@ -248,8 +248,8 @@
                                     <t t-foreach="staging.batch_ids"
                                        t-as="batch">
                                         <t t-set="first_pr"
-                                           t-value="batch.prs[-1]"/>
-                                        <li class="dropdown">
+                                           t-value="batch.prs[-1:]"/>
+                                        <li class="dropdown" t-if="first_pr">
                                             <button class="btn btn-link dropdown-toggle"
                                                     type="button"
                                                     data-toggle="dropdown"