Enable nullable (#2392)

* Enable nullable

* Several nullable fixes

* Fix dot-dsl

* calculator-conundrum: fix nullable

* developer-privileges: fix nullable

* face-id2: fix

* zebra-puzzle: fix

* pov: fix

* zipper: fix

* tournament: fix

* simple-linked-list: fix

* sgf-parsing: fix

* rest-api: fix

* react: fix

* robot-name: fix

* say: fix

* satellite: fix

* change: fix

* custom-set: fix

* grep: fix

* binary-search-tree: fix

* linked-list: fix

* alphametics: fix

* hangman: fix

* wizards-and-warriors-2: fix

* remote-control-cleanup: fix

* land-grab-in-space: fix

* instruments-of-texas: fix

* markdown: fix

* Dont remove nullable in new exercise

* More fixes

* remote-control: fix

[no important files changed]

exercises/concept/remote-control-competition/RemoteControlCompetition.cs

@@ -1,5 +1,3 @@
-
-
 public interface IRemoteControlCar
 {
     void Drive();