API: fix set milestone on PR creation (#14981)

* API: fix set milestone on PR creation

pr creation via API failed with 404, because we searched
for milestoneID 0, due to uninitialized var usage D:

* add tests

* fix expected status codes

* fix tests

Co-authored-by: 6543 <6543@obermui.de>

models/fixtures/label.yml

@@ -33,3 +33,11 @@
   num_issues: 1
   num_closed_issues: 0
   
+-
+  id: 5
+  repo_id: 10
+  org_id: 0
+  name: pull-test-label
+  color: '#000000'
+  num_issues: 0
+  num_closed_issues: 0