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>

patchwork/models.py

@@ -1079,8 +1079,11 @@ def _on_test_result(sender, result, **kwargs):
     if revision:
         series = revision.series
         user = series.submitter.user
+        revision_completed = "unknown"
+        if revision.completed:
+            revision_completed = revision.completed.isoformat()
         params = {'revision': revision.version,
-                  'revision_completed': revision.completed.isoformat()}
+                  'revision_completed': revision_completed}
     else:
         series = None
         user = patch.submitter.user