Fix incorrect $assertion parameter type of assert()

[FeBe95]

Changes the type of the $assertion parameter of the assert() function from Throwable|string|null to mixed. This follows the type that is stated in the official documentation:

Description

 assert(mixed $assertion, Throwable|string|null $description = null): bool

-- https://www.php.net/manual/en/function.assert.php

I guess this was a simple copy and paste error from when (with PHP 7.0.0) the second parameter $description was introduced, which actually is of type Throwable|string|null.