Replace the last NULL in our code with nullptr
Replace the last NULL in our code with nullptr

(The ones in filter/ don't count because they're autogenerated, and the
ones in test/catch.hpp are effectively in external library.)

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