Do not scale relays' bandwidth when there is not descriptor

Sometimes sbws does not get the descriptor for a relay that is in the consensus. This might be due legacy/trac#29716 (moved). I was hoping to make a refactor before changing this, but since refactoring would take longer, this should be fixed first.

Traceback (most recent call last):
  File "/usr/bin/sbws", line 11, in <module>
    load_entry_point('sbws==1.1.0.dev0', 'console_scripts', 'sbws')()
  File "/usr/lib/python3/dist-packages/sbws/sbws.py", line 75, in main
    exit(comm['f'](*comm['a'], **comm['kw']))
  File "/usr/lib/python3/dist-packages/sbws/core/generate.py", line 119, in main
    consensus_path=consensus_path)
  File "/usr/lib/python3/dist-packages/sbws/lib/v3bwfile.py", line 951, in from_results
    torflow_cap, round_digs)
  File "/usr/lib/python3/dist-packages/sbws/lib/v3bwfile.py", line 1248, in bw_torflow_scale
    desc_bw = min(desc_bw_obs, l.desc_bw_avg)
TypeError: '<' not supported between instances of 'NoneType' and 'NoneType'