Timeline: Show the "preview" only when the trace duration is invalid

The height of the trace is not sufficient to determine this as by
hiding categories it can be reduced to 0 without the trace being empty.

Change-Id: Ic8100b4da9c93808a6d49cf4ceba12de391ebc0e
Task-number: QTCREATORBUG-14277
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>