Guard against blank PR number being passed to test #3524

Job	Run time
Set CI/CD metadata	3s
Commit stage / Scan dependencies	29s
Commit stage / Scan secrets	8s
Test stage / Linting	41s
Test stage / Tests	1m 24s
Build stage / Build	22s
Deploy review app pr-746	0s
Post URL pr-${{ github.event.pull_request.number }} to PR comments	-1s
Test stage / Perform static analysis	7s
Test stage / Notifications End to End Tests	0s
Test stage / Notifications Integration Tests	0s
Test stage / Frontend Tests	0s
	3m 13s

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