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.

Most password managers used 100k until recently for the weaker sha256
version. We used 500k for the sha512 variant, so we should be good for a
while. Hopefully Matrix switches to argon2 or something better at that
point.

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