That DMV only tracks data since the last time SQL was started. If a DB has had no data modifications since the last time the server was started, that query will return null for that database.
SQL doesn't track modifications across server restarts, the best you'll get is when it was modified since the last time SQL started.
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