Markdown.pm6: Fixed warning when using invisible tags.
Markdown.pm6: Fixed warning when using invisible tags.

Warning:

    Use of Nil in string context  in block  at [...]/lib/Pod/To/Markdown.pm6:38

softmoth/perl6-pod-to-markdown,jpve/perl6-pod-to-markdown,softmoth/perl6-pod-to-markdown,zostay/perl6-pod-to-markdown,jpve/perl6-pod-to-markdown,zostay/perl6-pod-to-markdown