Checked for null before adding.

The [gulp docs](https://github.com/gulpjs/gulp/blob/master/docs/writing-a-plugin/guidelines.md) say: `If file.contents is null (non-read) just ignore the file and pass it along`. I ran into a situation like the following:

```
gulp.src(glob).pipe(filterMethod()).pipe(mocha());
```

And mocha was loading the files I was filtering out.