substr function, remove return false

[m-fedosov]

substr — Return part of a string

 substr(string $string, int $offset, ?int $length = null): string

Before PHP 8.0 if substr is empty, returns false

substr("abcdef", 4, -4);  // returns ""; prior to PHP 8.0.0, false was returned

In KPHP, because of this, you have to write additional conditions for translating false into an empty string. If this is fixed, the code becomes more readable and the behavior is more predictable.