Do not add a full keylist if no key is selected

* src/cryptcontroller.cpp (CryptController::lookup_fingerprints):
Handle the case no key is selected for a recipient.

This was a regression from the code to use recipient objects
for improved multikey selection for a recipient. In effect
it added the complete keyring as recipients if "No Key" was
selected as a recipient.

• GnuPG-Bug-Id: T5223