Don't write location coordinates if the underlying photo has broken location data

[alexwlchan]

Wikimedia Commons user HenkvD has spotted a fascinating bug: we wrote null coordinates in this edit.

The original photo is https://www.flickr.com/photos/ed_webster/16125227798/, which has busted location data:

<location latitude="0.000000" longitude="0.000000" accuracy="16" context="0">
  <neighbourhood woeid="0"/>
</location>

This looks like an easily detectable issue – we should just drop any location data from photos that appear to come from Null Island.

[alexwlchan]

This is now fixed on any new files we edit, and next time we fetch a snapshot we should check for any other files we've accidentally added null coordinates to.