APIServer Run method doesn't validates if Machines, Services or Units are present on the request.

[Environment]

Trusty 14.04.2
Juju-core 1.23.3

[Description]

The method apiclient.server.Run doesn't checks if Machines, Services or Units are
present on the request and this can lead to unexpected results.

A validation should be made just like the client is doing:

         if len(c.machines) == 0 && len(c.services) == 0 && len(c.units) == 0 {
             return fmt.Errorf("You must specify a target, either through --all, --machine, --service or --unit")
         }

This bug tracks that fix.