Another bugfix on get_from_uid LeObject method

When not found return None and do not try to run len() on None

lodel/leapi/leobject.py

         if len(res) > 1:
             raise LodelFatalError("Get from uid returned more than one \
 object ! For class %s with uid value = %s" % (cls, uid))
+        elif len(res) == 0:
+            return None
         return res[0]