test(user_ldap): return empty string for unset ldapUserDisplayName2 as per implementation

Signed-off-by: Josh <josh.t.richards@gmail.com>

Author: joshtrichards

apps/user_ldap/tests/AccessTest.php

@@ -708,7 +708,7 @@ public function testUserStateUpdate(): void {
 			->method('__get')
 			->willReturnMap([
 				[ 'ldapUserDisplayName', 'displayName' ],
-				[ 'ldapUserDisplayName2', null],
+				[ 'ldapUserDisplayName2', ''],
 			]);
 
 		$offlineUserMock = $this->createMock(OfflineUser::class);