Try to make unit test OK with PHP 7.4.

Author: niconoe-

tests/PHPStan/Analyser/nsrt/filter-var-returns-non-empty-string.php

@@ -211,7 +211,7 @@ public function run(
 	 * @param U $b
 	 * @return T|U
 	 */
-	private function anyOf($a, $b): mixed
+	private function anyOf($a, $b)
 	{
 		return random_int(0, 1) ? $a : $b;
 	}