A GROUP BY combined with aggregation should produce a single
row per group, except for empty groups. This is in contrast
to an aggregation without GROUP BY, which produces a single
row no matter what.
The existing code only considered the case of no grouping
and forced a row into the result, but this caused an unwanted
row if grouping was used.
Fix by refining the check to also consider GROUP BY.
XFAIL tests are relaxed.
Fixes#12477.
Note, forward_service requires that aggregation produce
exactly one row, but since it can't work with grouping,
it isn't affected.
Closes#14399
f6f974cdeb