feat(eventsub): Add support for channel.moderate Warn (#5932)

* fix(eventsub): Be less strict with strings & vectors

String foo from `{"foo":null}` is now accepted, and returns an empty string
std::vector<String> foo from `{"foo":null}` is now accepted, and returns an empty array

lib/twitch-eventsub-ws/src/string.cpp

@@ -11,6 +11,12 @@ boost::json::result_for<String, boost::json::value>::type tag_invoke(
     boost::json::try_value_to_tag<String> /*tag*/,
     const boost::json::value &jvRoot)
 {
+    if (jvRoot.is_null())
+    {
+        // We treat null "strings" as empty strings
+        return String("");
+    }
+
     auto v = boost::json::try_value_to<std::string>(jvRoot);
     if (v.has_error())
     {