support 7.1 nullable types in return type

[andares]

declare return type with nullable types is still crash.

like the code:

public function save(string $dir): ?string {
    return $dir ?: null;
}

[jens1o]

yep, because #292 is not merged, yet. Duplicate of #290.

@felixfbecker I hope it's okay for you to add labels...

[felixfbecker]

@jens1o fell free to tag issues and close duplicates