[GH-7982] Bugfix: Passing string|null DQL to Lexer(string $input) leads to downstream notice.

Author: beberlei

lib/Doctrine/ORM/Query/Parser.php

@@ -189,7 +189,7 @@ public function __construct(Query $query)
     {
         $this->query        = $query;
         $this->em           = $query->getEntityManager();
-        $this->lexer        = new Lexer($query->getDQL());
+        $this->lexer        = new Lexer((string) $query->getDQL());
         $this->parserResult = new ParserResult();
     }