Contextual filter Term OR Empty
You can draw some clues from this question: Views contextual filters for NULL value You should use Computed field or some other way of introducing custom PHP into your Views (custom module is the best option). 

If you use more than one Contextual and need to have the OR operator between them you can use the Views Contextual Filters OR module. 

There is also a patch for the D7 version you can expand on: Add an "OR IS NULL" option to contextual filters. 

But since you're using two vocabularies and need to maintain the AND between the two contextuals you won't be able to use the "OR" module and you'll have to come up with a custom code solution for the second contextual that takes care of the both "there is specific term" and "there are no terms" in one contextual filter, so you can combine it with the contextual for the Vocabulary 1.