Fix GH-13815: mb_trim() inaccurate $characters default value (#13820)

Because the default characters are defined in the stub file, and the
stub file is UTF-8 (typically), the characters

Fix GH-13815: mb_trim() inaccurate $characters default value (#13820)

Because the default characters are defined in the stub file, and the
stub file is UTF-8 (typically), the characters are encoded in the string
as UTF-8. When using a different character encoding, there is a mismatch
between what mb_trim expects and the UTF-8 encoded string it gets.

One way of solving this is by making the characters argument nullable,
which would mean that it always uses the internal code path that has the
unicode codepoints that are defaulted actually stored as codepoint
numbers instead of in a string.

Co-authored-by: @ranvis

show more ...