Extract user id from jwt sub claim

Author: BaggioGiacomo

lib/shopify_api/auth/jwt_payload.rb

@@ -47,9 +47,11 @@ def shop
       end
       alias_method :shopify_domain, :shop
 
-      sig { returns(Integer) }
+      sig { returns(T.nilable(Integer)) }
       def shopify_user_id
-        @sub.to_i
+        return unless @sub
+
+        @sub.tr("^0-9", "").to_i
       end
 
       alias_method :eql?, :==