Device management crashes on new NAV installation

[Morten Brekkevold]

Found this while testing the upcoming 3.3.0 release candidate. Probably,
device management is trying to retrieve the first alerthist entry to
determine which years to populate a dropdown box with. But, being a fresh
installation, there are no alerthist entries.

Traceback:

Mod_python error: "PythonHandler nav.web.devicemanagement.handler"

Traceback (most recent call last):

  File "/usr/lib/python2.4/site-packages/mod_python/apache.py", line 299,
in HandlerDispatch
    result = object(req)

  File "/usr/local/nav/lib/python/nav/web/devicemanagement/handler.py",
line 70, in handler
    output = history(req)

  File "/usr/local/nav/lib/python/nav/web/devicemanagement/history.py",
line 71, in history
    date_options['startyear'] = db.dictfetchall()[0]['min'].year

AttributeError: 'NoneType' object has no attribute 'year'

[http://sourceforge.net/tracker/index.php?func=detail&aid=1797010&group_id=107608&atid=648170]

[Stein Magnus Jodal (jodal)]

Originator: NO

Fixed in r4227:

Device Management:
* Aggregate functions like min() always returns a row, so one must check
if the
  value is NULL instead of checking for >0 rows. Somehow, the NULL value
in
  PostgreSQL translates to something not-None in Python, which has no
type,
  string representation but is True. This was solved by letting PSQL
decide if
  the returned value was true or not. (Closes: SF#1797010)