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/Alert/AlertOrderPrototype.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+#nullable enable
+using System.Collections.Generic;
 using Robust.Shared.Prototypes;
 using Robust.Shared.Serialization;
 using Robust.Shared.Utility;
@@ -12,7 +13,7 @@ namespace Content.Shared.Alert
     [Prototype("alertOrder")]
     public class AlertOrderPrototype : IPrototype, IComparer<AlertPrototype>
     {
-        public string ID { get; private set; }
+        public string ID { get; private set; } = string.Empty;
 
         private readonly Dictionary<AlertType, int> _typeToIdx = new();
         private readonly Dictionary<AlertCategory, int> _categoryToIdx = new();
@@ -23,7 +24,7 @@ namespace Content.Shared.Alert
 
             serializer.DataField(this, x => x.ID, "id", string.Empty);
 
-            if (!mapping.TryGetNode("order", out YamlSequenceNode orderMapping)) return;
+            if (!mapping.TryGetNode("order", out YamlSequenceNode? orderMapping)) return;
 
             var i = 0;
             foreach (var entryYaml in orderMapping)
@@ -56,7 +57,7 @@ namespace Content.Shared.Alert
             return -1;
         }
 
-        public int Compare(AlertPrototype x, AlertPrototype y)
+        public int Compare(AlertPrototype? x, AlertPrototype? y)
         {
             if ((x == null) && (y == null)) return 0;
             if (x == null) return 1;