lei completion: workaround old Perl bug
While `$argv[-1]' is `undef' on an empty @argv, using `$argv[-1]'
as a subroutine argument would fail incorrectly with:
Modification of non-creatable array value attempted, subscript -1 at ...
...even though we'd never attempt to modify @_ itself in the
subroutines being called. Work around the bug (tested on
5.16.3) by passing `undef' explicitly when `$argv[-1]' is
already `undef'.
Reported-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://public-inbox.org/meta/20210927124056.kj5okiefvs4ztk27@meerkat.local/