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/SharedStunnableComponent.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Threading;
 using Content.Shared.Alert;
@@ -40,7 +41,7 @@ namespace Content.Shared.GameObjects.Components.Mobs
         protected float KnockdownTimer;
         protected float SlowdownTimer;
 
-        private string _stunAlertId;
+        private string _stunAlertId = string.Empty;
 
         protected CancellationTokenSource StatusRemoveCancellation = new();
 
@@ -182,7 +183,7 @@ namespace Content.Shared.GameObjects.Components.Mobs
             SlowdownTimer = seconds;
             LastStun = _gameTiming.CurTime;
 
-            if (Owner.TryGetComponent(out MovementSpeedModifierComponent movement))
+            if (Owner.TryGetComponent(out MovementSpeedModifierComponent? movement))
                 movement.RefreshMovementSpeedModifiers();
 
             SetAlert();
@@ -191,7 +192,7 @@ namespace Content.Shared.GameObjects.Components.Mobs
 
         private void SetAlert()
         {
-            if (!Owner.TryGetComponent(out SharedAlertsComponent status))
+            if (!Owner.TryGetComponent(out SharedAlertsComponent? status))
             {
                 return;
             }