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

fixes https://github.com/Nheko-Reborn/nheko/issues/1218

This reverts commit a5bd5d00.