fix: Fix rules for shellcheck and shfmt jobs for when the version variable is not defined

shellcheck.yml

@@ -14,6 +14,7 @@ shellcheck:
     https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks/-/blob/main/shellcheck.md
     EOD
   rules:
-    - if: $GL_ASDF_SHELLCHECK_VERSION
+    - if: $GL_ASDF_SHELLCHECK_VERSION == null || $GL_ASDF_SHELLCHECK_VERSION == ""
+      when: never
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

shfmt.yml

@@ -17,6 +17,7 @@ shfmt:
     https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks/-/blob/main/shfmt.md
     EOD
   rules:
-    - if: $GL_ASDF_SHFMT_VERSION
+    - if: $GL_ASDF_SHFMT_VERSION == null || $GL_ASDF_SHFMT_VERSION == ""
+      when: never
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH