Crash on missing username

[shypike]

When converting a Trac dump from assembla.com, the migrator crashes on line 164:

Traceback (most recent call last):
  File "D:\Project\bzr\trac-launchpad-migrator-0.1.2\migrate.py", line 541, in <module>
    sys.exit(main(sys.argv[1:]))
  File "D:\Project\bzr\trac-launchpad-migrator-0.1.2\migrate.py", line 520, in main
    lp_comment = create_comment(db, comment)
  File "D:\Project\bzr\trac-launchpad-migrator-0.1.2\migrate.py", line 470, in create_comment
    trac_reporter = db.get_user(reporter)
  File "D:\Project\bzr\trac-launchpad-migrator-0.1.2\migrate.py", line 164, in get_user
    is_email = username.find('@') > -1
AttributeError: 'NoneType' object has no attribute 'find'

The cause is that the username is None (possibly a problem in the Trac export),
but the migrator should not just crash on this.

I "fixed" it by replacing line 164:
        is_email = username.find('@') > -1

with this line:
            is_email = username and username.find('@') > -1