Handle reading of empty YamlFiles

python-yaml's safe_load returns 'None' when it parses an empty YamlFile,
but since empty YamlFiles correspond to empty dictionaries, really, we
handle things accordingly.

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/storage/yaml_fs/yamlfile.py b/storage/yaml_fs/yamlfile.py
index 56d1325..7570155 100644
--- a/storage/yaml_fs/yamlfile.py
+++ b/storage/yaml_fs/yamlfile.py
@@ -20,7 +20,9 @@
 
     def _read(self):
         fp = file(self._path)
-        self._data = yaml.safe_load(fp)
+        data = yaml.safe_load(fp)
+        if data is not None:
+            self._data = data
         fp.close()
 
     def _get_entity(self):