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/AlertManager.cs

@@ -1,4 +1,6 @@
-using System.Collections.Generic;
+#nullable enable
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using Robust.Shared.IoC;
 using Robust.Shared.Log;
 using Robust.Shared.Prototypes;
@@ -13,12 +15,10 @@ namespace Content.Shared.Alert
         [Dependency]
         private readonly IPrototypeManager _prototypeManager = default!;
 
-        private Dictionary<AlertType, AlertPrototype> _typeToAlert;
+        private readonly Dictionary<AlertType, AlertPrototype> _typeToAlert = new();
 
         public void Initialize()
         {
-            _typeToAlert = new Dictionary<AlertType, AlertPrototype>();
-
             foreach (var alert in _prototypeManager.EnumeratePrototypes<AlertPrototype>())
             {
                 if (!_typeToAlert.TryAdd(alert.AlertType, alert))
@@ -34,7 +34,7 @@ namespace Content.Shared.Alert
         /// Tries to get the alert of the indicated type
         /// </summary>
         /// <returns>true if found</returns>
-        public bool TryGet(AlertType alertType, out AlertPrototype alert)
+        public bool TryGet(AlertType alertType, [NotNullWhen(true)] out AlertPrototype? alert)
         {
             return _typeToAlert.TryGetValue(alertType, out alert);
         }