Looking to the code at mongoUpdateContext() function, it seems that the only (theoretical) way  in which a zero-sized vector could be responsed is when the actionType is wrong, which should be detected early by the parsing layer before attempting invoking the mongoBackend.

All the other cases are threated by processContextElement() function which, as far as I checked, always results in a response vector greater than 0.

Thus, in sum, I think is safe to remove these fragments of code.

NTC