gen_sign_user_opts: fall back to default account if no account matches sender

gen_sign_user_opts: fall back to default account if no account matches sender

Fixes the following exception:

NoMethodError from thread: main
undefined method `gpgkey' for nil:NilClass
./lib/sup/crypto.rb:226:in `gen_sign_user_opts'
./lib/sup/crypto.rb:48:in `sign'
./lib/sup/util.rb:609:in `send'
./lib/sup/util.rb:609:in `method_missing'
./lib/sup/modes/edit-message-mode.rb:411:in `send'
./lib/sup/modes/edit-message-mode.rb:411:in `build_message'
./lib/sup/modes/edit-message-mode.rb:354:in `send_message'
./lib/sup/mode.rb:59:in `send'
./lib/sup/mode.rb:59:in `handle_input'
./lib/sup/buffer.rb:277:in `handle_input'
bin/sup:260

Signed-off-by: Sascha Silbe <sascha-pgp@silbe.org>