data_source_plugin: only keep in cache succesfull retrieval of a data source

portail_citoyen/apps/data_source_plugin/cms_plugins.py

@@ -83,7 +83,7 @@ class Data(object):
             else:
                 logger.debug('getting data source %r from url %r finished',
                         self.data_source.id, self.final_url)
-        if self.refresh:
+        if self.refresh and content is not None:
             cache.set(self.key, (content, self.now+self.refresh), 3600)
         return content