Handle an empty list of source files

[ruslansagitov]

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 function, on fs.stat(paths[complete], …), with:

Fatal error: path must be a string

Right now, “path” is undefined.

[tschaub]

Thanks for the fix @ruslansagitov. I've pulled this in as part of #63.