[fix] time_range in HTML form should not be a string 'None'

Before this patch SearXNG returns 'time_range': 'None'::

    <form method="GET" action="/searx/search">
      ...
      <input type="hidden" name="time_range" value="None">
      ...
    </form>

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>