fix: initialise loginName before isset check to avoid undefined-key warning

Signed-off-by: Thomas Müller <1005065+DeepDiver1975@users.noreply.github.com>

Author: DeepDiver1975

core/Controller/LoginController.php

@@ -165,12 +165,10 @@ public function showLoginForm($user, $redirect_url, $remember_login) {
 			// if the user exists, replace the userid with the username, e.g. for LDAP accounts
 			// that have the owncloud internal username set to a uuid.
 			$u = $this->userManager->get($user);
-			if ($u !== null) {
+			$parameters['loginName'] = $user;
+			if ($u !== null && \is_string($u->getUserName()) && $u->getUserName() !== '') {
 				$parameters['loginName'] = $u->getUserName();
 			}
-			if (!\is_string($parameters['loginName']) || $parameters['loginName'] === '') {
-				$parameters['loginName'] = $user;
-			}
 			$parameters['user_autofocus'] = false;
 		} else {
 			$parameters['loginName'] = '';