API: fix set milestone on PR creation (#14981) (#15001)

* 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

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

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

models/fixtures/repository.yml

@@ -146,6 +146,7 @@
   num_closed_issues: 0
   num_pulls: 1
   num_closed_pulls: 0
+  num_milestones: 1
   is_mirror: false
   num_forks: 1
   status: 0