…ok connector (elastic#198314)

Closes elastic#191915

## Summary

When editing an existing Webhook - Case Management connector clicking
Save will fail if the optional step 4 "Add a comment in case" is not
populated
- added extra checks for null, undefined or empty string: returns
"undefined" immediately, skipping any further validation - this ensures
that the "comment url" and "comment object" fields can remain empty
without triggering a validation error (previously, if the fields were
empty, the value passed was null).
- the header of the add comment section was changed to a toggle element,
when it's on, all the elements in the add comment section are visible

https://github.com/user-attachments/assets/49839173-e99b-4ee8-be6d-b682ad0b8a80

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Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>
(cherry picked from commit d278387)