shellwords only parses lines that are defined

[d-lamb]

Using perl 5.34.1 on MacOS with perl provided by Macports, the following produces a warning:

perl -MText::ParseWords -MDevel::CheckLib -E 'say $]; say Text::ParseWords->VERSION; say Devel::CheckLib->VERSION;'
Use of uninitialized value $line in substitution (s///) at /opt/local/lib/perl5/vendor_perl/5.34/Text/ParseWords.pm line 21.
5.034001
3.31
1.16

It's possible many modules (like Devel::CheckLib here) will accidentally call shellwords with an undefined value. The PR suppresses that warning by only doing the substitution if $line is defined in ParseWords.pm line 21.

I wanted to include a test in ParseWords.t, something like this:

eval {
    use warnings FATAL => qw(uninitialized);
    shellwords("a b c",undef,"d e f");
};
is($@,'','did not die on shellwords undef');

but I could not get make test to fail with the existing unpatched code. Perhaps you have a better idea.