workflows: Release Workflow - Avoid selecting random reviewers when no phab review

If you pass a NULL reivew ID to the differential.revision.search API,
it returns all revisions and we were just taking the first one and
assuming it was associated with the commit in the PR.

We need to exit early if we can't find a Phabricator review associated
with a commit.

Author: tstellar

llvm/utils/git/github-automation.py

@@ -99,6 +99,9 @@ def phab_get_commit_approvers(phab_token:str, repo:github.Repository.Repository,
     # API documentation: https://reviews.llvm.org/conduit/method/differential.parsecommitmessage/
     r = phab_api_call(phab_token, "https://reviews.llvm.org/api/differential.parsecommitmessage", args)
     review_id = r['result']['revisionIDFieldInfo']['value']
+    if not review_id:
+        # No Phabricator revision for this commit
+        return []
 
     args = {
         'constraints[ids][0]' : review_id,