fixed exception on no results

This should fix the exception we get when the remote command died without any output like this:

Traceback (most recent call last):
  File "/home/mozilla/src/steeplechase/steeplechase/runsteeplechase.py", line 308, in <module>
    sys.exit(0 if main(sys.argv[1:]) else 1)
  File "/home/mozilla/src/steeplechase/steeplechase/runsteeplechase.py", line 298, in main
    html_pass_count, html_fail_count = test.run()
  File "/home/mozilla/src/steeplechase/steeplechase/runsteeplechase.py", line 187, in run
    passes, failures = result
TypeError: 'NoneType' object is not iterable