LDAP synchronizations setup page accepts blank data for group cn and user filter

Summary

When using Groups > LDAP Synchronization it's possible to add empty synchronizations which show as an error (Inactive because syncing with an LDAP user filter is not included in the current license)

Steps to reproduce

• Go to Group > LDAP Synchronization groups/YOUR_GROUP/-/ldap_group_links
• Select LDAP Group cn or LDAP user filter
• Select Add Synchronization gives

What is the current bug behavior?

LDAP synchronization is saved on empty filters.

What is the expected correct behavior?

Make the LDAP group cn or LDAP user filter a required entry.

Output of checks

Results of GitLab environment info

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10.6.0

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)