Enable cache to work with a single resource

And an empty parent

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>

Author: pombredanne

src/commoncode/resource.py

@@ -604,20 +604,24 @@ def _use_disk_cache_for_resource(self, rid):
             msg = ['    Codebase._use_disk_cache_for_resource:, rid:', rid, 'mode:']
             if rid == 0:
                 msg.append('root')
+            elif rid is  None:
+                msg.append('from memory')
             elif self.all_on_disk:
                 msg.append('all_on_disk')
             elif self.all_in_memory:
                 msg.append('all_in_memory')
             else:
                 msg.extend(['mixed:', 'self.max_in_memory:', self.max_in_memory])
-                if rid < self.max_in_memory:
+                if rid and rid < self.max_in_memory:
                     msg.append('from memory')
                 else:
                     msg.append('from disk')
             logger_debug(*msg)
 
         if rid == 0:
             return False
+        elif rid is  None:
+            return False
         elif self.all_on_disk:
             return True
         elif self.all_in_memory: