DDL triggers are AFTER triggers, meaning they fire within the transaction *after* the changes have been made. So at that point, there is no record in sys.databases for the DB being dropped.
You'll have to work with the data that's in EVENTDATA, without going to sys.databases, for your trigger.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability