dispatcher: explicitly set missing interface argument for device actions

dispatcher: explicitly set missing interface argument for device actions

The first argument for dispatcher actions is the interface name, except
for the "hostname" action, where no interface is available.

For "hostname" actions continue to pass "none", as there is no interface
available. But for device actions, it may happen that the interface name
is missing too. In this case, don't pass "none" but instead an empty
name.