improvement: use null coalescing operator

[connorhu]

less code happier maintainer

The commit that has a rector prefix is the result of running the rector, the rest is manual.

[connorhu]

@thePanz

This kind of code has a simplified version:

        if (!isset(self::$instance)) {
            self::$instance = new sfAutoload();
        }

        return self::$instance;

        return self::$instance ?? self::$instance = new sfAutoload();

WDYT?