set_cheapest without checking pathlist

Hello,

Robert: I've taken the liberty of cc'ing you since you worked on most
of this code. My apologies if that wasn't appropriate.

While working on "Parallelize correlated subqueries that execute
within each worker" [1] I noticed that while in the other call to
set_cheapest (for partially_grouped_rel) in the same function the call
after gather_grouping_paths(root, partially_grouped_rel) is not
similarly guarded with a check for a NIL pathlist on
partially_grouped_rel.

I don't see any inherent reason why we must always assume that
gather_grouping_paths will always result in having at least one entry
in pathlist. If, for example, we've disabled incremental sort and the
cheapest partial path happens to already be sorted, then I don't
believe we'll add any paths. And if that happens then set_cheapest
will error with the message "could not devise a query plan for the
given query". So I propose we add a similar guard to this call point.

I could be convinced that this should be simply part of the patch in
the other thread, but it seemed to me it'd be worth considering
independently because as noted above I don't see any reason why this
couldn't happen separately. That being said, on master I don't have a
case showing this is necessary.

Thanks,
James Coleman

1: https://www.postgresql.org/message-id/flat/CAAaqYe-Aq6oNf9NPZnpPE7SgRLomXXWJA1Gz9L9ndi_Nv%3D94Yw%40mail.gmail.com#e0b1a803d0fdb97189ce493f15f99c14