Replace CanBeNull annotations with nullable reference types (#1270)

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

Content.Client/UserInterface/CreditsWindow.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -13,8 +14,6 @@ using Robust.Shared.Localization;
 using Robust.Shared.Maths;
 using Robust.Shared.Utility;
 
-#nullable enable
-
 namespace Content.Client.UserInterface
 {
     public sealed class CreditsWindow : SS14Window