fix: You can't insert before the first element with method insertBefore

Resolves: mirahman#4

Author: stanislavkrsv

DS/LinkedList/Classes/LinkedList.php

@@ -69,6 +69,11 @@ public function insertBefore(string $data = NULL, string $query = NULL) {
             while ($currentNode !== NULL) {
                 if ($currentNode->data === $query) {
                     $newNode->next = $currentNode;
+                    if ($previous === NULL) {
+                        $this->_firstNode = &$newNode;
+                    } else {
+                        $previous->next = $newNode;
+                    }
                     $previous->next = $newNode;
                     $this->_totalNode++;
                     break;