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/Audio/SoundCollectionPrototype.cs

@@ -1,4 +1,6 @@
+#nullable enable
 using System.Collections.Generic;
+using System.Data;
 using Robust.Shared.Prototypes;
 using Robust.Shared.Utility;
 using YamlDotNet.RepresentationModel;
@@ -8,21 +10,20 @@ namespace Content.Shared.Audio
     [Prototype("soundCollection")]
     public sealed class SoundCollectionPrototype : IPrototype
     {
-        public string ID { get; private set; }
-        public IReadOnlyList<string> PickFiles { get; private set; }
+        public string ID { get; private set; } = string.Empty;
+        public List<string> PickFiles { get; private set; } = new();
 
         public void LoadFrom(YamlMappingNode mapping)
         {
             ID = mapping.GetNode("id").AsString();
 
-            var pickFiles = new List<string>();
+            // In the unlikely case the method gets called twice
+            PickFiles.Clear();
 
             foreach (var file in mapping.GetNode<YamlSequenceNode>("files"))
             {
-                pickFiles.Add(file.AsString());
+                PickFiles.Add(file.AsString());
             }
-
-            PickFiles = pickFiles;
         }
     }
 }