[BUG] ipdevpoll psuwatch crash with TypeError immediately after upgrade to NAV 5

[lunkwill42]

Describe the bug

Shortly after upgrading to NAV 5, the power supply/fan data might not have been updated by the ipdevpoll inventory job, so the data is just what's was migrated with the schema migrations. This may lead to the psuwathc plugin attempting to monitor the status of a PSU or FAN unit whose internal_id has not yet been set.

Expected behavior

We cannot check the status of a unit whose ID is unkown. Their status should just be set to UNKNOWN as well.

Traceback

2019-12-10 11:48:49,968 [ERROR jobs.jobhandler] [statuscheck example-sw.example.org] Plugin nav.ipdevpoll.plugins.psuwatch.PowerSupplyOrFanStateWatcher('example-sw.example.org') reported an unhandled failure
Traceback (most recent call last):
  File "/opt/venvs/nav/lib/python3.5/site-packages/twisted/internet/defer.py", line 1532, in unwindGenerator
    return _inlineCallbacks(None, gen, Deferred())
  File "/opt/venvs/nav/lib/python3.5/site-packages/twisted/internet/defer.py", line 1386, in _inlineCallbacks
    result = g.send(result)
  File "/opt/venvs/nav/lib/python3.5/site-packages/nav/ipdevpoll/plugins/psuwatch.py", line 78, in _retrieve_current_unit_state
    state = yield method(unit.internal_id)
  File "/opt/venvs/nav/lib/python3.5/site-packages/twisted/internet/defer.py", line 1532, in unwindGenerator
    return _inlineCallbacks(None, gen, Deferred())
--- <exception caught here> ---
  File "/opt/venvs/nav/lib/python3.5/site-packages/twisted/internet/defer.py", line 1386, in _inlineCallbacks
    result = g.send(result)
  File "/opt/venvs/nav/lib/python3.5/site-packages/nav/mibs/cisco_entity_fru_control_mib.py", line 110, in get_power_supply_status
    "cefcFRUPowerOperStatus", (int(internal_id),)
builtins.TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'

Environment (please complete the following information):

• NAV version installed: 5.0.1