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/Prototypes/PDA/UplinkStoreListingPrototype.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using Content.Shared.GameObjects.Components.PDA;
 using Robust.Shared.Prototypes;
 using Robust.Shared.Serialization;
@@ -8,12 +9,12 @@ namespace Content.Shared.Prototypes.PDA
     [Prototype("uplinkListing")]
     public class UplinkStoreListingPrototype : IPrototype
     {
-        private string _id;
-        private string _itemId;
+        private string _id = string.Empty;
+        private string _itemId = string.Empty;
         private int _price;
         private UplinkCategory _category;
-        private string _desc;
-        private string _name;
+        private string _desc = string.Empty;
+        private string _name = string.Empty;
 
         public string ID => _id;