Add nil check for regex pattern in pattern AST

Author: josevalim

lib/elixir/lib/regex.ex

@@ -1008,6 +1008,9 @@ defmodule Regex do
 
     pattern_ast =
       cond do
+        is_nil(regex.re_pattern) ->
+          nil
+
         # TODO: Remove this when we require Erlang/OTP 28+
         # Before OTP 28.0, patterns did not contain any refs and could be safely be escaped
         :erlang.system_info(:otp_release) < [?2, ?8] ->