Bugfix for when one or more optional fields are blank. Also reduced logging quantity

I've tested this out with various combinations of missing (optional) fields so it should work properly now.

The one slight problem, however, is that redmine's issue model forces the "updated_on" field to be updated, so when the task saves the ticket uuid from Producer, the "updated_on" field is updated too. This is a potential annoyance in that it looks as if an issue has been updated when it hasn't (just synced), and potentially some tickets will not sync if they are created/modified whilst the rake task is running. The latter I could get around by adding a new field to the issue model - "last_synced", but the former problem, I don't know.