juju set/unset broken in trunk against 1.16

[John A Meinel]

juju set calls client.SetConstraints which uses the new Client.NewServiceSetForClientAPI because of the differences in handling of empty strings and null. (We want to support setting a config entry to the empty string, and passing Null to mean it should be reverted to the default value, or use juju unset for that.)

juju unset calls Client.ServiceUnset

However, those API calls weren't available in 1.16 so 1.18 would be incompatible with a 1.16 server.

A possible fix is to localize it in client.SetConstraints. (try the new API fall back to the old one if we can't get the new one, even though it might treat empty strings differently.)

Along the same lines *maybe* "juju unset" could pass the empty string to the old Client.SetConstraints?

part of bug #1253619