convert collections into immutable arrays

src/Discord.Net.Rest/Entities/Channels/RestThreadChannel.cs

@@ -81,7 +81,7 @@ namespace Discord.Rest
             Type = (ThreadType)model.Type;
             ParentChannelId = model.CategoryId.Value;
 
-            AppliedTags = model.AppliedTags.GetValueOrDefault(Array.Empty<ulong>());
+            AppliedTags = model.AppliedTags.GetValueOrDefault(Array.Empty<ulong>()).ToImmutableArray();
         }
 
         /// <summary>

src/Discord.Net.WebSocket/Entities/Channels/SocketThreadChannel.cs

@@ -153,7 +153,7 @@ namespace Discord.WebSocket
 
             HasJoined = model.ThreadMember.IsSpecified;
 
-            AppliedTags = model.AppliedTags.GetValueOrDefault(Array.Empty<ulong>());
+            AppliedTags = model.AppliedTags.GetValueOrDefault(Array.Empty<ulong>()).ToImmutableArray();
         }
 
         internal IReadOnlyCollection<SocketThreadUser> RemoveUsers(ulong[] users)