Use -n test instead of -z to determine dbt environment in workflow

.github/actions/configure_dbt_environment/action.yaml

@@ -6,9 +6,8 @@ runs:
     - name: Configure dbt environment
       run: |
         # GITHUB_HEAD_REF is only set on pull_request events, so if it's
-        # present, we must be in a PR context. Use the +x param expansion
-        # to distinguish empty strings: https://stackoverflow.com/a/13864829
-        if [ ! -z ${GITHUB_HEAD_REF+x} ]; then
+        # present, we must be in a PR context
+        if [ -n "$GITHUB_HEAD_REF" ]; then
           echo "On pull request branch, setting dbt env to CI"
           {
             echo "TARGET=ci";