fcrypt: skip empty key IDs as recipients/signers
fcrypt: skip empty key IDs as recipients/signers

I discovered this bug during my work on #1981 .
If an empty recipient/signature key ID is defined, GnuPG reports an error because it receives invalid command line arguments.

ElektraInitiative/libelektra,ElektraInitiative/libelektra,mpranj/libelektra,petermax2/libelektra,BernhardDenner/libelektra,mpranj/libelektra,ElektraInitiative/libelektra,ElektraInitiative/libelektra,BernhardDenner/libelektra,BernhardDenner/libelektra,BernhardDenner/libelektra,BernhardDenner/libelektra,BernhardDenner/libelektra,petermax2/libelektra,petermax2/libelektra,mpranj/libelektra,mpranj/libelektra,mpranj/libelektra,mpranj/libelektra,ElektraInitiative/libelektra,ElektraInitiative/libelektra,ElektraInitiative/libelektra,mpranj/libelektra,petermax2/libelektra,mpranj/libelektra,petermax2/libelektra,mpranj/libelektra,ElektraInitiative/libelektra,petermax2/libelektra,BernhardDenner/libelektra,ElektraInitiative/libelektra,ElektraInitiative/libelektra,petermax2/libelektra,mpranj/libelektra,BernhardDenner/libelektra,petermax2/libelektra,ElektraInitiative/libelektra,BernhardDenner/libelektra,petermax2/libelektra,mpranj/libelektra