Added nullable to most Content.Shared files (#3238)

* Add nullable to some Content.Shared files.

* Use [NotNullWhen(true)]

* Undo adding now redundant !'s

* Forgot one

* Add a ton more nullable

* You can guess

* Fix some issues

* It actually compiles now

* Auto stash before merge of "null2" and "origin/master"

* I lied

* enable annotations -> enable

* Revert ActionBlockerSystem.cs to original

* Fix ActionBlockerSystem.cs

* More nullable

* Undo some added exclamation marks

* Fix issues

* Update Content.Shared/Maps/ContentTileDefinition.cs

Co-authored-by: metalgearsloth <31366439+metalgearsloth@users.noreply.github.com>

* Resolve some issues

* Remove unused method

* Fix more issues

* Fix more issues

* Fix more issues

* Fix more issues

* Fix issue, rollback SharedGhostComponent.cs

* Update submodule

* Fix issue, invert some if-statements to reduce nesting

* Revert RobustToolbox

* FIx things broken by merge

* Some fixes

- Replaced with string.Empty
- Remove some exclamation marks
- Revert file

* Some fixes

* Trivial #nullable enable

* Fix null ables

Co-authored-by: metalgearsloth <31366439+metalgearsloth@users.noreply.github.com>
Co-authored-by: Metal Gear Sloth <metalgearsloth@gmail.com>

Content.Shared/GameObjects/Components/Mobs/State/SharedCriticalMobState.cs

@@ -1,4 +1,5 @@
-using Content.Shared.Alert;
+#nullable enable
+using Content.Shared.Alert;
 using Content.Shared.GameObjects.EntitySystems;
 using Robust.Shared.GameObjects;
 
@@ -15,7 +16,7 @@ namespace Content.Shared.GameObjects.Components.Mobs.State
         {
             base.EnterState(entity);
 
-            if (entity.TryGetComponent(out SharedAlertsComponent status))
+            if (entity.TryGetComponent(out SharedAlertsComponent? status))
             {
                 status.ShowAlert(AlertType.HumanCrit); // TODO: combine humancrit-0 and humancrit-1 into a gif and display it
             }