Fix overwriting package.json repository

[JelteMX]

When using the generator, you can update the development files of an existing widget (run yo mendix in the root of the widget). This uses a template. Unfortunately, it does not ask whether you have a repository defined in the package.json. It the automatically adds this to this files, but Githubuser is undefined. This results in:

  "repository": {
    "type": "git",
    "url": "http://github.com/undefined/<widgetName>"
  },

(So widgetName is defined, but the Github user is not)