protocol: Add explicit nullable types

Most of the time it does not make sense to pass a NULL object, string, or array
to a protocol request. This commit adds an explicit “allow-null” attribute
to mark the request arguments where NULL makes sense.

Passing a NULL object, string, or array to a protocol request which is not
marked as allow-null is now an error. An implementation will never receive
a NULL value for these arguments from a client.

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>