Protect github.get_content() against empty result

`get_content()` may currently crash, e.g. right now on `http://ghtrendingrss.appspot.com/rss?language=arc&timespan=monthly`
Relevant trace is the following:

```
  File "/base/data/home/apps/s~ghtrendingrss/1.378450970759818941/githubrss.py", line 23, in get_rss
    rss_feed = self.rssproducer.get_rss(languageId, timespanId)
  File "/base/data/home/apps/s~ghtrendingrss/1.378450970759818941/githubrss.py", line 23, in get_rss
    rss_feed = self.rssproducer.get_rss(languageId, timespanId)
  File "/base/data/home/apps/s~ghtrendingrss/1.378450970759818941/githubrss.py", line 9, in get_rss
    rss_feed = rss.get_rss(github.languages[languageId], github.timespans[timespanId], github.get_projects(languageId, timespanId))
  File "/base/data/home/apps/s~ghtrendingrss/1.378450970759818941/github.py", line 39, in get_projects
    } for project_raw in projects_raw]
  File "/base/data/home/apps/s~ghtrendingrss/1.378450970759818941/github.py", line 18, in get_content
    return bsobject.string.strip()
AttributeError: 'NoneType' object has no attribute 'strip'
```
This PR should fix it.