Fix PHP8.2 str_split function returns empty arrays for empty strings

[HuongNV13]

In PHP 8.2, the str_split function will returns empty arrays for empty strings.
See: https://php.watch/versions/8.2/str_split-empty-string-empty-array

We can use mb_str_split() instead

[google-cla]

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[bshaffer]

I'm not sure why exactly, but something is different in mb_str_split that is causing all our tests to fail

[HuongNV13]

Hi @bshaffer,
The failed tests are related to the CLA.
I have signed the CLA and rebased the code.

[SoulPancake]

@HuongNV13 CLA has passed but the test suites are still failing as you can see, Can you check once again?

[bshaffer]

The two functions behave differently when dealing with a binary string, which is the case here:

str_split() will split into bytes, rather than characters when dealing with a multi-byte encoded string. Use mb_str_split() to split the string into code points.

If there's a specific error you're trying to prevent, perhaps you can do it by verifying the input beforehand.