Don't apply the group filter when listing LDAP group membership if it is empty (#23745) (#23788)

Backport #23745 by @zeripath

When running listLdapGroupMemberships check if the groupFilter is empty
before using it to list memberships.

Fix #23615

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: zeripath <art27@cantab.net>

services/auth/source/ldap/source_search.go

@@ -208,7 +208,7 @@ func (source *Source) listLdapGroupMemberships(l *ldap.Conn, uid string, applyGr
 	}
 
 	var searchFilter string
-	if applyGroupFilter {
+	if applyGroupFilter && groupFilter != "" {
 		searchFilter = fmt.Sprintf("(&(%s)(%s=%s))", groupFilter, source.GroupMemberUID, ldap.EscapeFilter(uid))
 	} else {
 		searchFilter = fmt.Sprintf("(%s=%s)", source.GroupMemberUID, ldap.EscapeFilter(uid))