fix: enable the shellcheck and shfmt jobs only when their respective version are defined

shellcheck.yml

@@ -14,5 +14,6 @@ shellcheck:
     https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks/-/blob/main/shellcheck.md
     EOD
   rules:
-    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    - if: $GL_ASDF_SHELLCHECK_VERSION
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

shfmt.yml

@@ -17,5 +17,6 @@ shfmt:
     https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks/-/blob/main/shfmt.md
     EOD
   rules:
-    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    - if: $GL_ASDF_SHFMT_VERSION
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH