Ignore buildkite branch when not in a PR (#3180) (#3183)

(cherry picked from commit 96be7a2)

Co-authored-by: Miguel Grinberg <miguel.grinberg@gmail.com>

Author: github-actions[bot]

test_elasticsearch/test_server/test_rest_api_spec.py

@@ -500,7 +500,7 @@ def remove_implicit_resolver(cls, tag_to_remove):
     http = urllib3.PoolManager(retries=urllib3.Retry(total=10))
 
     branch_candidates = []
-    if "ES_YAML_TESTS_BRANCH" in os.environ:
+    if "ES_YAML_TESTS_BRANCH" in os.environ and os.environ["ES_YAML_TESTS_BRANCH"]:
         branch_candidates.append(os.environ["ES_YAML_TESTS_BRANCH"])
     git_branch = subprocess.getoutput("git branch --show-current")
     if git_branch not in branch_candidates: