Fix a parsing problem when last_name is null.

Hi!!

When using your script to convert from bugzilla to redmine, I found an issue. And you are referenced on the official redmine page, http://www.redmine.org/projects/redmine/wiki/RedmineMigrate, I send you my small contribution.

- Fix a parsing problem when last_name is null after converting from real_name.

Hope it helps !!