define empty request_key method for sumstats in cluster mode.

This prevents the worker nodes from crashing, when request_key is used
in cluster mode and called on the worker and the manager nodes (i.e. when
a non-cluster-aware script is used).

Addresses BIT-1177
Merge remote-tracking branch 'origin/topic/bernhard/ticket1177'

* origin/topic/bernhard/ticket1177:
  define empty request_key method for sumstats in cluster mode.