Eliminate undef warnings.

Makefile.PL

@@ -19,6 +19,7 @@ test_requires  'Test::More'      => '0.42';
 test_requires  'Test::Exception' => undef;
 test_requires  'List::MoreUtils' => undef;
 test_requires 'Test::Differences' => undef;
+test_requires 'Test::NoWarnings' => undef;
 test_requires  'FindBin'         => undef;
 
 if ($Module::Install::AUTHOR) {

lib/Text/Markdown.pm

@@ -266,6 +266,9 @@ sub urls {
 sub _CleanUpDoc {
     my ($self, $text) = @_;
 
+    # An empty file can end up getting passed as undef.
+    return "\n\n" unless defined $text;
+
     # Standardize line endings:
     $text =~ s{\r\n}{\n}g;  # DOS to Unix
     $text =~ s{\r}{\n}g;    # Mac to Unix

t/05empty_markdown.t

@@ -0,0 +1,9 @@
+use strict;
+use warnings;
+use Test::More tests => 3;
+use Text::Markdown;
+use Test::NoWarnings;
+
+my $m = Text::Markdown->new;
+ok $m->markdown(''), 'Parse empty string';
+ok $m->markdown(undef), 'Parse undef';