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/Construction/ConstructionGraphEdge.cs

@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using System.Collections.Generic;
 using Content.Shared.Interfaces;
 using Robust.Shared.IoC;
@@ -13,11 +14,11 @@ namespace Content.Shared.Construction
     public class ConstructionGraphEdge : IExposeData
     {
         private List<ConstructionGraphStep> _steps = new();
-        private List<IEdgeCondition> _conditions;
-        private List<IGraphAction> _completed;
+        private List<IEdgeCondition> _conditions = new();
+        private List<IGraphAction> _completed = new();
 
         [ViewVariables]
-        public string Target { get; private set; }
+        public string Target { get; private set; } = string.Empty;
 
         [ViewVariables]
         public IReadOnlyList<IEdgeCondition> Conditions => _conditions;
@@ -48,7 +49,7 @@ namespace Content.Shared.Construction
             var serializer = YamlObjectSerializer.NewReader(mapping);
             InternalExposeData(serializer);
 
-            if (!mapping.TryGetNode("steps", out YamlSequenceNode stepsMapping)) return;
+            if (!mapping.TryGetNode("steps", out YamlSequenceNode? stepsMapping)) return;
 
             foreach (var yamlNode in stepsMapping)
             {