There are other cases where topics without titles can be created.
I migrated from vBulletin 3 years ago, and profile posts were imported into Discourse as titleless topics.
Maybe discourse/lib/topic_creator.rb at 24feb20abc713d06106a480b0b4e2f20b3938360 · discourse/discourse · GitHub lacks this verification? I didn’t check it as I barely know how to read code and didn’t investigate the vBulletin migration script either. It’s just a guess.