Write empty GPX file when there's no GPS data

[pe-st]

This basically a port of my PR kjkjava#34 to your fork. Here's a copy of its description:

I had this error for a couple of activities:

Garmin Connect activity: [436081459] La Berra
        Sun, 26 Jan 2014 10:00, 05:14:09, 9.25 Kilometers
        Downloading file...
Traceback (most recent call last):
  File "./gcexport.py", line 215, in <module>
    data = http_req(download_url)
  File "./gcexport.py", line 74, in http_req
    raise Exception('Bad return code (' + str(response.getcode()) + ') for: ' + url)
Exception: Bad return code (204) for: https://connect.garmin.com/modern/proxy/download-service/export/gpx/activity/436081459

I fixed it by writing an empty GPX file in this case