[Perl/perl5] 358f5b: Perl__byte_dump_string(): Properly handle NULL input

[Karl Williamson via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: 358f5bbe54cdef533446eee53fb5a7d073a721f6
      
https://github.com/Perl/perl5/commit/358f5bbe54cdef533446eee53fb5a7d073a721f6
  Author: Karl Williamson <k...@cpan.org>
  Date:   2023-07-16 (Sun, 16 Jul 2023)

  Changed paths:
    M embed.fnc
    M proto.h
    M utf8.c

  Log Message:
  -----------
  Perl__byte_dump_string(): Properly handle NULL input

Instead of an assert, this returns (nil) and things can proceed.