Ensure source_root value is not undefined

This prevents an issue I found when running against a Meteor instance of `0.6.6.3` where the `self.source_root` value was no longer being returned from `self`.

The `filePath` that was being loaded was complete, so I just suppressed the error by ensuring it was an empty-string, not `undefined`.

You may have a more sensible thing you want to do here.

Thanks.