Actually ignore devices with no driver configured

get_devices() logs that it ignores but does skip the device: it is still indexed by switch_id or switch_info and returned to the mechanism driver, which passes the None driver name to stevedore. The result is that neutron-server fails to start with “TypeError: sequence item 0: expected str instance, NoneType found”, which says nothing about the option that is missing.

Add the missing continue, and one to the switch_id/switch_info branch so its intent is explicit rather than relying on nothing being indexed when neither is set.