Replace CanBeNull annotations with nullable reference types (#1270)

Co-authored-by: Pieter-Jan Briers <pieterjan.briers+git@gmail.com>

Content.Client/ClickMapManager.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Text;
@@ -11,8 +12,6 @@ using Robust.Shared.ViewVariables;
 using SixLabors.ImageSharp;
 using SixLabors.ImageSharp.PixelFormats;
 
-#nullable enable
-
 namespace Content.Client
 {
     internal class ClickMapManager : IClickMapManager, IPostInjectInit