message: [bugfix] empty From date filter NoneType error
  Error "AttributeError: 'NoneType' object has no attribute 'split'" is
  fired.
diff: @@ -158,6 +158,7 @@ class BaseViz(object):
  until = form_data.get("until", "now")
  # Backward compatibility hack
  + if since:
  since_words = since.split(' ')
  grains = ['days', 'years', 'hours', 'day', 'year', 'weeks']
  if (len(since_words) == 2 and since_words[1] in grains):
  