Handle editing new files.

`editor.getUri()` returns `undefined` when editing a new file which causes a TypeError when trying to resolve the path. This change prevents the path errors; however, the default (or home) configuration will be used.

Atom doesn't currently have a way to get the editor's working directory when editing a new file (`editor.getPath()` also returns `undefined`) and `process.cwd()` returns `'/'` which doesn't resolve to allow the local .jshintrc to be found.

There's room for improvement here, but this will prevent the error that currently occurs.