Quay config app LDAP filter should not be mandatory field

Description:
This is an issue found when configuring Quay to use LDAP authentication, in Quay 3.3 with new feature LDAP filter introduced, user can provide valid LDAP filter, but this field should not be mandatory, the current behavior is if users leave this field as empty, Quay config app can't save the configurations.

Steps:
1. Open Quay config app
2. Click "Modify configurations for this cluster"
3. At "Internal Authentication" section, select LDAP
4. Input valid LDAP configurations, leave "Additional User Filter Expression:" as empty
5. Click "Save Configurations"

Actual Result:
The configurations was NOT saved successfully.

Expected Result:
The configurations was saved successfully.