When varlink parameters are empty, omit the "parameters" field entirely
rather than sending "parameters":{}. This reduces message size and
follows varlink specification which allows parameters to be omitted.
The implementation supports three equivalent representations for empty
parameters: field omission, JSON null, and empty object {}. All three
are accepted on input for backward compatibility.
Fixes: #38474
We have this in a similar fashion for the other APIs libsystemd
provides. Add the same for sd-varlink. There isn't too much on it for
now, but at least it's a start.
Also link it up everywhere.