Resolve "Fix validation on External Wiki service template form"

What does this MR do?

In Administration → Service Templates → External Wiki the form always requires a URL, while the form in Project Settings → Integrations → External Wiki only requires a URL if the service is enabled. This MR fixes the form in the administration to do the same.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Issue: #20748 (closed)