The validation behaviors of filter field and Relative DN field of LDAP setting are not consistent

Description of problem:

When using quay config web console to config LDAP Authentication, set an invalid filter setting to make no user matching, the validae configuration changes process will through out a error as `Could not find any users matching filter in LDAP server`. But when set a invalid `Relative DN` setting which make no user matching, there is no validation error, the validation pass. The behaviors of these two cases are not consistent.

Version-Release number of selected component (if applicable):

quay 3.6.2

How reproducible:

always

Steps to Reproduce:
1. log in quay config web console to set `Internal Authentication` as LDAP
2. type valid values for other required fields
3. type an invalid value in `Additional User Filter Expression` fields
4. click `validae configuration changes` button to check the error message
5. clear up `Additional User Filter Expression` field
6. type an invalid value in `Relative DN` fields
7. click `validae configuration changes` button to check the error message

Actual results:

the validation results of invalid `Additional User Filter Expression` setting and invalid `Relative DN` setting are not consistent. If there no user matching due to invalid `Additional User Filter Expression` setting, customer can get a error message `Could not find any users matching filter in LDAP server`. But if there no user matching due to invalid `Relative DN` setting, the validation pass and without error message.

Expected results:

the validation behaviors of these two cases should be consistent, through out error message `Could not find any users matching filter in LDAP server`