core (memory/cache): geFilePath now return None when uid is empty

src/memory/cache.py

@@ -57,6 +57,9 @@ class Cache(object):
         @return (unicode, None): absolute path to cached file
             None if file is not in cache (or cache is invalid)
         """
+        uid = uid.strip()
+        if not uid:
+            return None
         cache_url = self.getPath(uid)
         if not os.path.exists(cache_url):
             return None