Handle invalid types in schedule pipeline form

The following discussion from !184694 (merged) should be addressed:

• @arfedoro started a discussion:

  suggestion (non-blocking) 🏓

  What do you think about having some fallback values for invalid types?

  

Implementation

• frontend use placeholder when the cadence type is invalid in ee/app/assets/javascripts/security_orchestration/components/policy_editor/pipeline_execution/rule/schedule_form.vue