squash! Improve PHPUnit assertions

Make PHPUnit assertion of an array-key of an actual array redundant
condition free on evaluation and reconciliation of an actual array to
the array type in the psalm-assertion.

stubs/Assert_75.phpstub

@@ -8,7 +8,7 @@ abstract class Assert
      *
      * @param int|string $key
      * @param mixed $array
-     * @psalm-assert array $array
+     * @psalm-assert array{} $array
      * @return void
      */
     public static function assertArrayHasKey($key, $array, string $message = '') {}