add nonce integer check

lib/block/validation/local/nonce.ex

@@ -3,6 +3,6 @@ defmodule Block.Validation.Local.Nonce do
   https://git.duniter.org/documents/rfcs/-/blob/master/rfc/0010_Duniter_Blockchain_Protocol_V12.md#nonce
   """
   def valid(block) do
-    block["nonce"] >= 0
+    is_integer(block["nonce"]) and block["nonce"] >= 0
   end
 end