Replace CanBeNull annotations with nullable reference types (#1270)

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

Content.IntegrationTests/Tests/Networking/SimplePredictReconcileTest.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
@@ -15,8 +16,6 @@ using Robust.Shared.IoC;
 using Robust.Shared.Map;
 using Robust.Shared.Timing;
 
-#nullable enable
-
 namespace Content.IntegrationTests.Tests.Networking
 {
     // This test checks that the prediction & reconciling system is working correctly with a simple boolean flag.