This is required by the spec, however no such condition is actually
present in the rules... To work around that, we just add our own, but we
should write an MSC to fix that eventually.

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.