Do not use a default when getting the buildkite PR branch (#3185) (#3188)

(cherry picked from commit 3eb91b0)

Co-authored-by: Miguel Grinberg <[email protected]>

Author: github-actions[bot]

.buildkite/run-tests

@@ -9,7 +9,7 @@ export STACK_VERSION="${STACK_VERSION:=8.0.0-SNAPSHOT}"
 export TEST_SUITE="${TEST_SUITE:=platinum}"
 export PYTHON_VERSION="${PYTHON_VERSION:=3.14}"
 export PYTHON_CONNECTION_CLASS="${PYTHON_CONNECTION_CLASS:=urllib3}"
-export ES_YAML_TESTS_BRANCH=${BUILDKITE_PULL_REQUEST_BASE_BRANCH:=main}
+export ES_YAML_TESTS_BRANCH="${BUILDKITE_PULL_REQUEST_BASE_BRANCH}"
 
 script_path=$(dirname $(realpath $0))
 source $script_path/functions/imports.sh