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/Preferences/Appearance/HairStyles.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;

Content.Shared/Preferences/Appearance/HumanoidCharacterAppearance.cs

@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using Content.Shared.GameObjects.Components.Body.Part;
 using Robust.Shared.Serialization;
 

Content.Shared/Preferences/BackpackPreference.cs

@@ -1,3 +1,4 @@
+#nullable enable
 namespace Content.Shared.Preferences
 {
     /// <summary>

Content.Shared/Preferences/ClothingPreference.cs

@@ -1,3 +1,4 @@
+#nullable enable
 namespace Content.Shared.Preferences
 {
     /// <summary>

Content.Shared/Preferences/GameSettings.cs

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

Content.Shared/Preferences/HumanoidCharacterAppearance.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Linq;
 using Content.Shared.Preferences.Appearance;

Content.Shared/Preferences/ICharacterAppearance.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 namespace Content.Shared.Preferences
 {
     public interface ICharacterAppearance

Content.Shared/Preferences/ICharacterProfile.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 namespace Content.Shared.Preferences
 {
     public interface ICharacterProfile

Content.Shared/Preferences/JobPriority.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 namespace Content.Shared.Preferences
 {
     public enum JobPriority

Content.Shared/Preferences/PlayerPreferences.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Collections.Generic;
 using Robust.Shared.Maths;
@@ -45,12 +46,6 @@ namespace Content.Shared.Preferences
 
         public Color AdminOOCColor { get; set; }
 
-        public int FirstEmptySlot()
-        {
-            var firstEmpty = IndexOfCharacter(null);
-            return firstEmpty == -1 ? _characters.Count : firstEmpty;
-        }
-
         public int IndexOfCharacter(ICharacterProfile profile)
         {
             return _characters.FirstOrNull(p => p.Value == profile)?.Key ?? -1;

Content.Shared/Preferences/PreferenceUnavailableMode.cs

@@ -1,3 +1,5 @@
+#nullable enable
+
 namespace Content.Shared.Preferences
 {
     /// <summary>