Add empty formaction to formaction_stack instead of a nullptr
Add empty formaction to formaction_stack instead of a nullptr

Fixes https://github.com/newsboat/newsboat/issues/1094

newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat