Earned Value = Activity Progress x  Planned or Baseline value (To which value to be fetched in order to calculate the Earned value is decided on the option we give when creating the Baseline) 

If a Baseline has not been created, the system will always fetch the Planned value to calculate the Earned Value. 

There will always be an earned value in a Sub Project or in an Activity as long as there is Activity Progress and Baseline/Planned Cost. 

If we give 'Planned' for Earned Value Method when creating the Baseline and if there isn't any Planned cost for the Activity, there will not be an Earned Value for that Activity even though there is a progress in the Activity. 

Hope explanation will help on you !