aggregator: don't produce empty/uninitialised results
aggregator: don't produce empty/uninitialised results

Empty buckets are part of the startup, and also in the longer run of the
process, likely don't mean much.

lyft/carbon-c-relay,tjamesturner/carbon-c-relay,zzl0/carbon-c-relay,grobian/carbon-c-relay,lyft/carbon-c-relay,piotr1212/carbon-c-relay,rtoma/carbon-c-relay,piotr1212/carbon-c-relay,grobian/carbon-c-relay,Skyscanner/carbon-c-relay,piotr1212/carbon-c-relay,Skyscanner/carbon-c-relay,zzl0/carbon-c-relay,lyft/carbon-c-relay,grobian/carbon-c-relay,rtoma/carbon-c-relay,piotr1212/carbon-c-relay,tjamesturner/carbon-c-relay