[Perl/perl5] 6bdaa5: utf8::upgrade: Don't coerce undef arg

[Karl Williamson via perl5-changes]

  Branch: refs/heads/smoke-me/khw-upgrade
  Home:   https://github.com/Perl/perl5
  Commit: 6bdaa5242c2aa2b0d42d1af144bbd90e775c96dc
      
https://github.com/Perl/perl5/commit/6bdaa5242c2aa2b0d42d1af144bbd90e775c96dc
  Author: Karl Williamson <k...@cpan.org>
  Date:   2022-11-02 (Wed, 02 Nov 2022)

  Changed paths:
    M universal.c

  Log Message:
  -----------
  utf8::upgrade: Don't coerce undef arg

This fixes GH #20419