Null key in timeseries result for hyperUnique on missing columns

[gianm]

If the column for a hyperUnique aggregator doesn't exist, a null key appears in the TimeseriesResultValue. If results get combined by TimeseriesBinaryFn, these null keys get filtered out. But if results don't get combined (possibly because only one segment matched the query) then the query fails with a Jackson exception when it tries to serialize the null key, because null keys aren't allowed in JSON.

[gianm]

I think I can fix this as part of #3572 since the underlying cause of the null key is that Aggregators.noopAggregator has no name.

[gianm]

Fixed in #3572; test case added in #3630. Even though the latter isn't merged yet I'll close this since the bug is fixed.