Allows us to implement extra functionality and shortens the symbol
names. Does not work with gcc10.

If an Unknown message is being constructed for sending, the content
may be empty, since the msgtype and body are exposed as C++ properties.
Previously, the to_json function assumed that the content could be
parsed as valid json, when in fact an empty content would throw an error.

Signed-off-by: Vitaly Zaitsev <vitaly@easycoding.org>

It makes no sense for servers to send the parent object without events
and not sending an empty list, but strictly speaking that is spec
compliant, so...

Relates to https://github.com/Nheko-Reborn/nheko/issues/1257

* Add confetti support