nip10: handle invalid reply-with-no-root

I noticed a few clients do this even though its not valid. Let's handle it.

Signed-off-by: William Casarin <jb55@jb55.com>

damus/NIP10/ThreadReply.swift

@@ -48,6 +48,11 @@ struct ThreadReply {
             }
         }
 
+        // reply with no root should be considered reply-to-root
+        if root == nil && reply != nil {
+            root = reply
+        }
+
         // nip10 threads must have a root
         guard let root else {
             return nil