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/Atmos/AtmosDirection.cs

@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using System.Runtime.CompilerServices;
 using Robust.Shared.Maths;
 using Robust.Shared.Serialization;

Content.Shared/Atmos/Atmospherics.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using Robust.Shared.Maths;
 using Robust.Shared.Serialization;
 using System;

Content.Shared/Atmos/GasPrototype.cs

@@ -1,4 +1,6 @@
-using Robust.Shared.Prototypes;
+#nullable enable
+using System;
+using Robust.Shared.Prototypes;
 using Robust.Shared.Serialization;
 using YamlDotNet.RepresentationModel;
 
@@ -7,12 +9,12 @@ namespace Content.Shared.Atmos
     [Prototype("gas")]
     public class GasPrototype : IPrototype
     {
-        public string Name { get; private set; }
+        public string Name { get; private set; } = string.Empty;
 
         // TODO: Control gas amount necessary for overlay to appear
         // TODO: Add interfaces for gas behaviours e.g. breathing, burning
 
-        public string ID { get; private set; }
+        public string ID { get; private set; } = string.Empty;
 
         /// <summary>
         ///     Specific heat for gas.
@@ -43,25 +45,25 @@ namespace Content.Shared.Atmos
         /// <summary>
         ///     If this reagent is in gas form, this is the path to the overlay that will be used to make the gas visible.
         /// </summary>
-        public string GasOverlayTexture { get; private set; }
+        public string GasOverlayTexture { get; private set; } = string.Empty;
 
         /// <summary>
         ///     If this reagent is in gas form, this will be the path to the RSI sprite that will be used to make the gas visible.
         /// </summary>
-        public string GasOverlayState { get; set; }
+        public string GasOverlayState { get; set; } = string.Empty;
 
         /// <summary>
         ///     State for the gas RSI overlay.
         /// </summary>
-        public string GasOverlaySprite { get; set; }
+        public string GasOverlaySprite { get; set; } = string.Empty;
 
         /// <summary>
         /// Path to the tile overlay used when this gas appears visible.
         /// </summary>
-        public string OverlayPath { get; private set; }
+        public string OverlayPath { get; private set; } = string.Empty;
 
 
-        public string Color { get; private set; }
+        public string Color { get; private set; } = string.Empty;
 
         public void LoadFrom(YamlMappingNode mapping)
         {