Provide better error handling when files are empty

To reproduce

• create a light module called e.g. my-module
• create an empty page template definition at my-module/templates/pages/super-page.yam (this can easily happen if a dev doesn't use the create-page command)
• run mgnl add-availability my-module:components/my-comp my-module:pages/super-page@main -p my-module/
  The following error is thrown, which is probably not very useful for an average CLI user

  path.js:7
      throw new TypeError('Path must be a string. Received ' + inspect(path));
      ^
  TypeError: Path must be a string. Received null
      at assertPath (path.js:7:11)
      at Object.join (path.js:1251:7)
      at modifyYamlConfiguration (/Users/fgrilli/devel/workspace/magnolia/build/npm-cli/bin/addAvailability.js:71:35)
      at /Users/fgrilli/devel/workspace/magnolia/build/npm-cli/bin/addAvailability.js:65:5
      at tryToString (fs.js:414:3)
      at FSReqWrap.readFileAfterClose [as oncomplete] (fs.js:401:12)