Add null checks in RegisterSidToTypeConnection

Author: JaThePlayer

Celeste.Mod.mm/Mod/Registry/EntityRegistry.cs

@@ -27,6 +27,9 @@ public static class EntityRegistry {
     public static IReadOnlySet<string> GetKnownSidsFromType(Type type) => TypeToSids.GetValueOrDefault(type) ?? EmptyStringSet;
 
     internal static void RegisterSidToTypeConnection(string sid, Type type) {
+        if (sid is null || type is null)
+            return;
+
         ref var sidToTypeEntry = ref CollectionsMarshal.GetValueRefOrAddDefault(SidToTypes, sid, out _);
         sidToTypeEntry ??= new(1);
         sidToTypeEntry.Add(type);
@@ -270,4 +273,4 @@ static EntityRegistry() {
         RegisterSidToTypeConnection("spawnFacingTrigger", typeof(SpawnFacingTrigger));
         RegisterSidToTypeConnection("detachFollowersTrigger", typeof(DetachStrawberryTrigger));
     }
-}
+}