Replace CanBeNull annotations with nullable reference types (#1270)

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

Content.Server/GameObjects/Components/Access/AccessReaderComponent.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -10,8 +11,6 @@ using Robust.Shared.Interfaces.GameObjects;
 using Robust.Shared.Serialization;
 using Robust.Shared.ViewVariables;
 
-#nullable enable
-
 namespace Content.Server.GameObjects.Components.Access
 {
     /// <summary>
@@ -68,7 +67,6 @@ namespace Content.Server.GameObjects.Components.Access
             return _accessLists.Count == 0 || _accessLists.Any(a => a.IsSubsetOf(accessTags));
         }
 
-        [CanBeNull]
         public static ICollection<string> FindAccessTags(IEntity entity)
         {
             if (entity.TryGetComponent(out IAccess accessComponent))