Options are now all global but may be overridden per-output. If an
output local value is requested but none has been set, the global value
is provided instead. This makes for much better ergonomics when
configuring layout related options in particular.
Passing an empty string as the value argument for riverctl set-option or
declare-option will set the value to null. The riverctl get-option
command produces no output for both null and empty string values.
This is not perfect as it is unable to distinguish between null and
empty strings through the riverctl CLI. I don't see a better alternative
here however. Forbidding null strings in the river-options protocol
would be one solution, however null strings are useful and more pleasant
to use from code despite being problematic on the CLI.
