Make empty string a valid extension option

I have a use case of gulp-swig where I use a `.swig` extension on the input files...something like `template.html.swig`.  I would like gulp-swig to be able to just remove the extension entirely--i.e. use an empty string for `opts.ext`.  An empty string, however, is falsy, and so this pull request addresses my use case.  The current behavior is mostly unchanged, as the default `.html` extension is still applied if `opts.ext` is `null` or `undefined`.