HOTFIX: Check if there are samples before processing them

Otherwise Pending projects or any project without samples won't load.

```
Traceback (most recent call last):
  File "/Users/guillem/anaconda/envs/master/lib/python2.7/site-packages/tornado-3.2-py2.7-macosx-10.5-x86_64.egg/tornado/web.py", line 1218, in _when_complete
    callback()
  File "/Users/guillem/anaconda/envs/master/lib/python2.7/site-packages/tornado-3.2-py2.7-macosx-10.5-x86_64.egg/tornado/web.py", line 1239, in _execute_method
    self._when_complete(method(*self.path_args, **self.path_kwargs),
  File "/Users/guillem/repos/status/status/projects.py", line 250, in get
    self.write(json.dumps(self.project_info(project)))
  File "/Users/guillem/repos/status/status/projects.py", line 273, in project_info
    summary_row.value.update(self.sample_progress_rows(samples))
  File "/Users/guillem/repos/status/status/projects.py", line 173, in sample_progress_rows
    for sample_name, sample_data in sample_rows.iteritems():
AttributeError: 'NoneType' object has no attribute 'iteritems'
```