Catch case, if value to update with is None

[white-gecko]

If a node tag without timestamp is parsed a TypeError is thrown.

<node id='-25388' action='modify' visible='true' lat='1.234567' lon='9.876543' />

Traceback (most recent call last):
  File "…/.local/bin/map-machine", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "…/map-machine/map_machine/main.py", line 44, in main
    mapper.render_map(arguments)
  File "…/map-machine/map_machine/mapper.py", line 367, in render_map
    osm_data.parse_osm_file(input_file_name)
  File "…/map-machine/map_machine/osm/osm_reader.py", line 446, in parse_osm_file
    self.parse_osm(ElementTree.parse(file_name).getroot())
  File "…/map-machine/map_machine/osm/osm_reader.py", line 479, in parse_osm
    self.add_node(node)
  File "…/map-machine/map_machine/osm/osm_reader.py", line 368, in add_node
    self.time.update(node.timestamp)
  File "…/map-machine/map_machine/util.py", line 18, in update
    self.min_ = value if not self.min_ or value < self.min_ else self.min_
                                          ^^^^^^^^^^^^^^^^^
TypeError: '<' not supported between instances of 'NoneType' and 'datetime.datetime'

This pullrequest adds a check for None.