allow tests to pass with Dist::Zilla::Role::TestRunner < 5.014

e.g.
http://www.cpantesters.org/cpan/report/1f0e7608-6bfb-1014-a820-ef492d84075e
length(undef) warns on <5.12
Dist-Zilla-Plugin-MakeMaker-Awesome-0.28

   - fix test failure caused by unstated assumption that
     Dist::Zilla::Role::TestRunner is a very recent version
   - fix uninitialized warning on 5.10.x and earlier