don't walk through empty dirs

...otherwise you'd get
```
Using Dist::Inktly::Minty to create Foo::Bar...
Error opendir on '/home/sromanov/perl5/xt': No such file or directory at /home/sromanov/perl5/perlbrew/perls/perl-5.10.1/lib/site_perl/5.10.1/Dist/Inktly/Minty.pm line 181.
```