Behavior of topn when missing time ranges

when doing a query for say 1am to 12am, if there are segments missing for a timerange, eg 10-11, what is the behavior of topn?
does it:

  1. return 10-11 timerange anyway, but with a ‘value’ of ‘0’?

  2. 10-11 is not even present in the output?

Does the same behavior apply to other queries like timeseries, groupby, etc?