Fix undefined array key error

Author: tvdijen

src/Auth/Source/Negotiate.php

@@ -215,7 +215,11 @@ public function authenticate(array &$state): void
                     }
 
                     if (($lookup = $this->lookupUserData($uid)) !== null) {
-                        if ($state['core:username'] !== null && strcasecmp($lookup['userPrincipalName'][0], $state['core:username']) !== 0) {
+                        if (
+                            array_key_exists('core:username', $state)
+                            && $state['core:username'] !== null
+                            && strcasecmp($lookup['userPrincipalName'][0], $state['core:username']) !== 0
+                        ) {
                             // The login-hint doesn't match the already authenticated user. Fall back.
                             $this->fallBack($state);
                             return;