Fix handling of str_split / mb_str_split string arg compound types + default return type for mb_str_split

[herndlm]

Closes phpstan/phpstan#7580

There were 2 issues basically which is why I split this into 2 commits

• str_split / mb_str_split did not handle compound types for the string type correctly (e.g. union of constant strings)
• mb_str_split can actually return an empty array. This was adapted in mb_str_split() / str_split() always returns non-empty-array #633 which is based on a psalm adaption, but psalm actually reverted it back in Revert mb_str_split return signature to list<string> vimeo/psalm#6493 because str_split and mb_str_split differ in behaviour there. See also https://3v4l.org/r6sMe

StrSplitFunctionReturnTypeExtension could need a bit more cleanup, but I wanted to avoid rewriting it here.. Maybe I'll adapt some more in follow-ups.

[mvorisek]

FYI, str_split will return empty array for empty string since PHP 8.2, see php/php-src#8945

[herndlm]

FYI, str_split will return empty array for empty string since PHP 8.2, see php/php-src#8945

oh good to know and it least it's consistent then. I'd adapt that in a follow-up if that's ok :) UPDATE: after your linked PR gets merged of course

[ondrejmirtes]

Thank you!