Fix error in handling receipts
Fix error in handling receipts

Fixes an error which has been happening ever since #2158 (v0.21.0-rc1):

> TypeError: argument of type 'ObservableDeferred' is not iterable

fixes #3234

matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse