add null check for stat https://github.com/docpad/docpad-plugin-tags/issues/3

when using the tags plugin, this line throws sometimes an uncaught exception because `stat` is null  
fixes https://github.com/docpad/docpad-plugin-tags/issues/3

node v0.10.24
npm 1.3.21
docpad: 6.60.3
docpad-plugin-tags 2.0.7