add test

Author: staabm

tests/PHPStan/Rules/Variables/NullCoalesceRuleTest.php

@@ -362,4 +362,9 @@ public function testBug14213(): void
 		]);
 	}
 
+	public function testBug13921(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13921.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Variables/data/bug-13921.php

@@ -0,0 +1,9 @@
+<?php
+
+namespace Bug13921;
+
+/** @param list<array<?string>> $x */
+function foo(array $x): void {
+	var_dump($x[0]['bar'] ?? null);
+	var_dump($x[0] ?? null);
+}