Currently underlying ES aggregations are returned as are, both as sub-aggregations and bucket selector.
Unfortunately due to the underlying inconsistencies this can have undesirable effects, for example:

1. MAX/MIN can work on date fields however they return an actual long instead of a time object causing failures
2. aggregations on null/empty groups return their initial result, not null which again is problematic (see Introduce contract to know whether InternalAggs have computed results or not #34903).

In both example, this can be handled inside SQL by wrapping the aggregation to guarantee the expected value.