fix undefined warnings

lib/WebGUI/Asset.pm

@@ -180,6 +180,7 @@ around extraHeadTags => sub {
     if (@_ > 0) {
         my $unpacked = $_[0];
         my $packed   = $unpacked;  ##Undo magic aliasing since a reference is passed below
+        return if !defined $packed;
         HTML::Packer::minify( \$packed, {
             remove_newlines     => 1,
             do_javascript       => "shrink",