Handle varieties of empty <georss:point> element

Empty point defined as <georss:point> </georss:point> with single space only leads to index out of range on the coordinates access.

From the log:

```
ERROR:planet.runner:Error processing http://example.com/feed
ERROR:planet.runner:IndexError: list index out of range
ERROR:planet.runner:  File "/opt/venus/planet/spider.py", line 480, in spiderPlanet
    writeCache(uri, feed_info, data)
ERROR:planet.runner:  File "/opt/venus/planet/spider.py", line 228, in writeCache
    xdoc = reconstitute.reconstitute(data, entry)
ERROR:planet.runner:  File "/opt/venus/planet/reconstitute.py", line 315, in reconstitute
    location(xentry, (float)(coordinates[1]), (float)(coordinates[0]))
```