init: Fix queue_all_property_triggers with nonexistent properties

Don't queue actions for "on property=*" if the property does not exist.
This fixes these errors on boot:

    init: property 'sys.powerctl' doesn't exist while
    expanding '${sys.powerctl}'
    init: powerctl: cannot expand '${sys.powerctl}

Change-Id: I3bd354d73a860f856be5df2c654f940445f9efd9