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/AlertPrototype.cs

@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using System.Globalization;
 using Content.Shared.Interfaces;
 using Robust.Shared.IoC;
@@ -17,7 +18,7 @@ namespace Content.Shared.Alert
     [Prototype("alert")]
     public class AlertPrototype : IPrototype
     {
-        public string ID { get; private set; }
+        public string ID { get; private set; } = string.Empty;
 
         /// <summary>
         /// Type of alert, no 2 alert prototypes should have the same one.
@@ -31,17 +32,17 @@ namespace Content.Shared.Alert
         /// to get the correct icon path for a particular severity level.
         /// </summary>
         [ViewVariables]
-        public SpriteSpecifier Icon { get; private set; }
+        public SpriteSpecifier Icon { get; private set; } = SpriteSpecifier.Invalid;
 
         /// <summary>
         /// Name to show in tooltip window. Accepts formatting.
         /// </summary>
-        public FormattedMessage Name { get; private set; }
+        public FormattedMessage Name { get; private set; } = new();
 
         /// <summary>
         /// Description to show in tooltip window. Accepts formatting.
         /// </summary>
-        public FormattedMessage Description { get; private set; }
+        public FormattedMessage Description { get; private set; } = new();
 
         /// <summary>
         /// Category the alert belongs to. Only one alert of a given category
@@ -85,7 +86,7 @@ namespace Content.Shared.Alert
         /// Defines what to do when the alert is clicked.
         /// This will always be null on clientside.
         /// </summary>
-        public IAlertClick OnClick { get; private set; }
+        public IAlertClick? OnClick { get; private set; }
 
         public void LoadFrom(YamlMappingNode mapping)
         {
@@ -198,7 +199,7 @@ namespace Content.Shared.Alert
             return AlertType == other.AlertType && AlertCategory == other.AlertCategory;
         }
 
-        public override bool Equals(object obj)
+        public override bool Equals(object? obj)
         {
             return obj is AlertKey other && Equals(other);
         }