Option "default" has been added to file.readJSON.

Before it, in case when file is not exists or empty some Exception raised.
But now, if option "default" passed (only if passed) it returned if file not exists or empty instead exception.

Useful case:
We try to override some options from json file that can not exists. And now we are writing construction:
overwrite: grunt.file.exists('app_dev/manifest.json') ? grunt.file.readJSON('app_dev/manifest.json') : {}

But with new option "default" it will be like:
overwrite: grunt.file.readJSON('app_dev/manifest.json', {default:{}})