UngroupedAggregateRegionScanner.insertEmptyKeyValue() generates too many cells

UngroupedAggregateRegionScanner.insertEmptyKeyValue() is supposed to generate one empty cell for each timestmap.
However, it never adds the generated timestamp to the processed map, and generates an empty cell for each existing cell.
IUC, these are all sent to the the RS and written to WAL, and are only consolidated in the memstore, leading to a lot of wasted cpu, memory, io, and network resources.