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/Disposal/SharedDisposalMailingUnitComponent.cs

@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using System.Collections.Generic;
 using Robust.Shared.GameObjects;
 using Robust.Shared.Serialization;
@@ -37,10 +38,10 @@ namespace Content.Shared.GameObjects.Components.Disposal
             public readonly bool Engaged;
             public readonly string Tag;
             public readonly List<string> Tags;
-            public readonly string Target;
+            public readonly string? Target;
 
             public DisposalMailingUnitBoundUserInterfaceState(string unitName, string unitState, float pressure, bool powered,
-                bool engaged, string tag, List<string> tags, string target)
+                bool engaged, string tag, List<string> tags, string? target)
             {
                 UnitName = unitName;
                 UnitState = unitState;
@@ -57,7 +58,7 @@ namespace Content.Shared.GameObjects.Components.Disposal
                 return new DisposalMailingUnitBoundUserInterfaceState(UnitName, UnitState, Pressure, Powered, Engaged, Tag, (List<string>)Tags.Clone(), Target);
             }
 
-            public bool Equals(DisposalMailingUnitBoundUserInterfaceState other)
+            public bool Equals(DisposalMailingUnitBoundUserInterfaceState? other)
             {
                 if (other is null) return false;
                 if (ReferenceEquals(this, other)) return true;
@@ -91,9 +92,9 @@ namespace Content.Shared.GameObjects.Components.Disposal
         [Serializable, NetSerializable]
         public class UiTargetUpdateMessage : BoundUserInterfaceMessage
         {
-            public readonly string Target;
+            public readonly string? Target;
 
-            public UiTargetUpdateMessage(string target)
+            public UiTargetUpdateMessage(string? target)
             {
                 Target = target;
             }