Replace assertion with exception

This issue can actually happen.

src/Query/Parser.php

@@ -2924,7 +2924,10 @@ public function ArithmeticPrimary()
             return new AST\ParenthesisExpression($expr);
         }
 
-        assert($this->lexer->lookahead !== null);
+        if ($this->lexer->lookahead === null) {
+            $this->syntaxError('ArithmeticPrimary');
+        }
+
         switch ($this->lexer->lookahead->type) {
             case TokenType::T_COALESCE:
             case TokenType::T_NULLIF:

tests/Tests/ORM/Functional/Ticket/GH11487Test.php

@@ -0,0 +1,40 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\Tests\ORM\Functional\Ticket;
+
+use Doctrine\ORM\Mapping\Column;
+use Doctrine\ORM\Mapping\Entity;
+use Doctrine\ORM\Mapping\GeneratedValue;
+use Doctrine\ORM\Mapping\Id;
+use Doctrine\ORM\Query\QueryException;
+use Doctrine\Tests\OrmFunctionalTestCase;
+
+class GH11487Test extends OrmFunctionalTestCase
+{
+    public function testItThrowsASyntaxErrorOnUnfinishedQuery(): void
+    {
+        $this->expectException(QueryException::class);
+        $this->expectExceptionMessage('Syntax Error');
+        $this->_em->createQuery('UPDATE Doctrine\Tests\ORM\Functional\Ticket\TaxType t SET t.default =')->execute();
+    }
+}
+
+/** @Entity */
+class TaxType
+{
+    /**
+     * @var int|null
+     * @Column(type="integer")
+     * @Id
+     * @GeneratedValue
+     */
+    public $id;
+
+    /**
+     * @var bool
+     * @Column(type="boolean")
+     */
+    public $default = false;
+}