fix: sanitize empty variable test expressions

rewrite all '$SOME_VAR' test expressions to '($SOME_VAR != null && $SOME_VAR != "")'

templates/validation.yml

@@ -60,7 +60,7 @@ gitlab-ci-lint:
     - ci_lint
   rules:
     # enabled if $GITLAB_TOKEN and GITLAB_CI_FILES are set
-    - if: '$GITLAB_TOKEN && $GITLAB_CI_FILES'
+    - if: '($GITLAB_TOKEN != null && $GITLAB_TOKEN != "") && $GITLAB_CI_FILES'
 
 check-links:
   image: