Combine function returning NULL unhandled?

Hi,

Since
commit a7de3dc5c346e07e0439275982569996e645b3c2
Author: Robert Haas <rhaas@postgresql.org>
Date:   2016-01-20 13:46:50 -0500
   Support multi-stage aggregation.      Aggregate nodes now have two new modes: a "partial" mode where they   output
theunfinalized transition state, and a "finalize" mode where   they accept unfinalized transition states rather than
individual  values as input.      These new modes are not used anywhere yet, but they will be necessary   for parallel
aggregation. The infrastructure also figures to be   useful for cases where we want to aggregate local data and remote
data via the FDW interface, and want to bring back partial aggregates   from the remote side that can then be combined
withlocally generated   partial aggregates to produce the final value.  It may also be useful   even when neither FDWs
norparallelism are in play, as explained in   the comments in nodeAgg.c.      David Rowley and Simon Riggs, reviewed by
KaiGaiKohei, Heikki   Linnakangas, Haribabu Kommi, and me.
 

there's both advance_transition_function and advance_combine_function,
fulfilling closely related duties.

While working on that code (making transition and combine functions go
through expression evaluation, so they can be JITed), I noticed a small
difference in behaviour beteween the two:

The plain transition case contains:    if (pergroupstate->transValueIsNull)    {        /*         * Don't call a
strictfunction with NULL inputs.  Note it is         * possible to get here despite the above tests, if the transfn is
      * strict *and* returned a NULL on a prior cycle. If that happens         * we will propagate the NULL all the way
tothe end.         */        return;    }
 

how come similar logic is not present for combine functions? I don't see
any checks preventing a combinefunc from returning NULL, nor do I see
https://www.postgresql.org/docs/devel/static/sql-createaggregate.html
spell out a requirement that that not be the case.

Greetings,

Andres Freund