Python 3 port: do not compare None with an integer

On Python 2, that is allowed.  On Python 3, it raises a TypeError
with "'>' not supported between instances of 'NoneType' and 'int'".
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/core/runner.py

@@ -403,7 +403,7 @@ class TestRunner(object):
         timeout = float(timeout or self.DEFAULT_TIMEOUT)
 
         test_deadline = time_started + timeout
-        if job_deadline > 0:
+        if job_deadline is not None and job_deadline > 0:
             deadline = min(test_deadline, job_deadline)
         else:
             deadline = test_deadline