cpp: Go with default answer on unknown question by key edit interface

* lang/cpp/src/editinteractor.cpp (edit_interactor_callback_impl):
Send empty string to edit interface if General Error occurred.

A General Error is usually returned by the nextState function of the
concrete EditInteractor subclasses if gpg asks an unexpected question
which isn't handled by the edit interactor's state machine. In this
case, it's usually safe to go with the default answer. This makes the
edit interactors much more robust.

• GnuPG-bug-id: T7274