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.Server/GameObjects/Components/Disposal/DisposalMailingUnitComponent.cs

@@ -95,10 +95,10 @@ namespace Content.Server.GameObjects.Components.Disposal
         private readonly List<string> _targetList = new();
 
         [ViewVariables]
-        private string _target = "";
+        private string? _target;
 
         [ViewVariables(VVAccess.ReadWrite)]
-        private string _tag = "";
+        private string _tag = string.Empty;
 
         [ViewVariables]
         public bool Powered =>
@@ -288,6 +288,11 @@ namespace Content.Server.GameObjects.Components.Disposal
                 _container.Remove(entity);
             }
 
+            if (_target == null)
+            {
+                return false;
+            }
+
             var holder = CreateTaggedHolder(entities, _target);
 
             entryComponent.TryInsert(holder);
@@ -441,7 +446,7 @@ namespace Content.Server.GameObjects.Components.Disposal
                 }
             }
 
-            if (obj.Message is UiTargetUpdateMessage tagMessage && TagRegex.IsMatch(tagMessage.Target))
+            if (obj.Message is UiTargetUpdateMessage tagMessage && TagRegex.IsMatch(tagMessage.Target ?? string.Empty))
             {
                 _target = tagMessage.Target;
             }
@@ -714,6 +719,11 @@ namespace Content.Server.GameObjects.Components.Disposal
 
         bool IInteractHand.InteractHand(InteractHandEventArgs eventArgs)
         {
+            if (eventArgs.User == null)
+            {
+                return false;
+            }
+
             if (!eventArgs.User.TryGetComponent(out IActorComponent? actor))
             {
                 return false;