Fix get module name from empty string

I fixed a bug on getting the module name from empty files
Sorry, trailing whitespace.
You might consider to install an [EditorConfig](http://editorconfig.org/) plugin for your favorite text-editor.
It would pick up our config [.editconfig](https://github.com/D-Programming-Language/dub/blob/master/.editorconfig) and avoid any indentation/spacing/tabs/newline issues.