Don't skip threads with empty id tags during migration

The assumption of the comment is false. The id tag of a thread can be empty on non-empty threads, too! (At least this is the case for the Disqus export file I exported recently.)

In my data, all id tags are empty, which caused isso to skip _all_ threads.

Removing these lines fixed the bug. After the change all comments were imported successfully.

This patch fixes #135.