fix for empty file

```
gulp.task('prefix', function() {
    var prefixUrl = 'http://mydomain.com/asset';

    gulp.src('empty.html')
        .pipe(prefix(prefixUrl, null, true))
        .pipe(gulp.dest('build'));
});
```

```
Error: File.contents can only be a Buffer, a Stream, or null.
```

i create a empty file by `touch`, and gulp-prefix broken...