Handle an empty list of source files

There’s a possibility that the list of source files could be empty. Review:
```
taskName: {
    src: 'src/**/*.js',
    dest: 'build/dist.js'
}
```
If there’re no `*.js` files in the `src` directory, grunt-newer fails in the [anyNewer](https://github.com/tschaub/grunt-newer/blob/70badaab692cb6d445758a7a39678404cb9cfb24/lib/util.js#L53) function, on `fs.stat(paths[complete], …)`, with:
```
Fatal error: path must be a string
```
Right now, “path” is undefined.