[Perl/perl5] d932d8: 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: d932d836e84badcbf7ccf9605ec030016b880d69
      
https://github.com/Perl/perl5/commit/d932d836e84badcbf7ccf9605ec030016b880d69
  Author: Karl Williamson <k...@cpan.org>
  Date:   2022-11-05 (Sat, 05 Nov 2022)

  Changed paths:
    M lib/utf8.t
    M universal.c

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

This fixes GH #20419