Validate pipeline schedule variable key

What does this MR do and why?

Ensures pipeline schedules cannot be created without a variable key.

Screenshots or screen recordings

How to set up and validate locally

1. Try creating a pipeline schedule with no key to ensure it's not created.
2. Try creating a pipeline schedule with key/value to ensure it is created.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383870 (closed)