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/SharedWiresComponent.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Diagnostics.CodeAnalysis;
 using JetBrains.Annotations;
@@ -115,12 +116,12 @@ namespace Content.Shared.GameObjects.Components
         public class WiresBoundUserInterfaceState : BoundUserInterfaceState
         {
             public string BoardName { get; }
-            public string SerialNumber { get; }
+            public string? SerialNumber { get; }
             public ClientWire[] WiresList { get; }
             public StatusEntry[] Statuses { get; }
             public int WireSeed { get; }
 
-            public WiresBoundUserInterfaceState(ClientWire[] wiresList, StatusEntry[] statuses, string boardName, string serialNumber, int wireSeed)
+            public WiresBoundUserInterfaceState(ClientWire[] wiresList, StatusEntry[] statuses, string boardName, string? serialNumber, int wireSeed)
             {
                 BoardName = boardName;
                 SerialNumber = serialNumber;