The only thing that has worked for me in keeping the RadMaskedEditBox empty when no value is entered by the user is to set the MinimumDate value to a date I know will not be used, say 1/1/1900, and then check for this date value in the ValueChanged event for my DateTimePicker control. If that date is the value, I reset the DateTimePicker to NULL, using the SetToNullValue() method, and set the text value of the control to an empty string, "" in VB.
The check looks like this for one of my controls:
udtDtofDeath.Value = #1/1/1900#
This is the only way I have found that keeps the RadMaskedEditBox blank when a user inadvertently clicks on it or tabs into it and does not enter a value of their own.