bot: add rule to auto-merge on no code changes

On pull request with no code changes, the canary and integration tests
won't run. This means they won't appear in the status and mergify cannot
validate them.

Signed-off-by: Sébastien Han <seb@redhat.com>

.mergify.yml

@@ -18,7 +18,7 @@ pull_request_rules:
         message: This pull request has merge conflicts that must be resolved before it can be merged. @{{author}} please rebase it. https://rook.io/docs/rook/master/development-flow.html#updating-your-fork
 
   # automerge on master only under certain strict conditions
-  - name: automerge merge master with specific label and approvals
+  - name: automerge merge master with specific label and approvals with code change
     conditions:
       - author=mergify[bot]
       - base=master
@@ -48,6 +48,25 @@ pull_request_rules:
       dismiss_reviews: {}
       delete_head_branch: {}
 
+  - name: automerge merge master with specific label and approvals without code change
+    conditions:
+      - author=mergify[bot]
+      - base=master
+      - "#approved-reviews-by>=1"
+      - "#changes-requested-reviews-by=0"
+      - label!=do-not-merge
+      - label=ready-to-merge
+      - 'check-success=lint'
+      - 'check-success=DCO'
+      - 'check-success=continuous-integration/jenkins/pr-head'
+      - 'check-success=build'
+    actions:
+      merge:
+        method: merge
+        strict: false
+      dismiss_reviews: {}
+      delete_head_branch: {}
+
   # automerge backports if CI successfully ran
   # release-1.3 branch
   - name: automerge backport release-1.3