models: Fix revision date for older series

Old series do not have a completion date for series which breaks posting
results:

  File "patchwork/models.py" in _on_test_result
    1083.                   'revision_completed': revision.completed.isoformat()}

  Exception Type: AttributeError at /api/1.0/series/71839/revisions/1/test-results/
  Exception Value: 'NoneType' object has no attribute 'isoformat'

Let's just admit that it's unknown.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
1 job for master in 1 minute and 22 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #1337045
test

00:01:22