Replace CanBeNull annotations with nullable reference types (#1270)

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

Content.Server/GameObjects/Components/PDA/PDAComponent.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using System;
 using System.Collections;
 using System.Collections.Generic;
@@ -22,8 +23,6 @@ using Robust.Shared.Localization;
 using Robust.Shared.Serialization;
 using Robust.Shared.ViewVariables;
 
-#nullable enable
-
 namespace Content.Server.GameObjects.Components.PDA
 {
     [RegisterComponent]