Fixed DocParser lookahead to work in PHP 7.4

[davidbyoung]

Fixes #283 by explicitly checking if the lookahead position is null before using it (started becoming a problem in PHP 7.4). Also fixed spl autoload function to have a void return type rather than bool. Removed an error message from the PHPStan config because it is no longer thrown by the fixed DocParser

[malarzm]

We should rather merge #276 up as it's doing mostly the same thing and is already in 1.7 branch

[greg0ire]

So we should just merge up? Is this repo using a merge up policy?

[malarzm]

@greg0ire I assumed it does although I don't know how much master diverged from 1.7

[alcaeus]

I'll merge up soon. I wanted to fix the 1.7 vs. 1.8 issue first (some PRs were marked for 1.7 but only merged to master), then solve the issue by merging up to master.

For what it's worth, we use merge-up strategy here as it requires less work (merging up is generally easier and less error-prone than backporting). As such I'm closing this PR, but I'll leave the related issue open until I've done the merge.