fix: Allow a correlated scalar subquery to group without aggregating

[Dandandan]

Which issue does this PR close?

• Closes #.

Rationale for this change

A correlated scalar subquery must return at most one row per set of outer
values. check_aggregation_in_scalar_subquery required an aggregate expression
to establish that, so this query fails to plan:

select o.id, (select k from dim where dim.k = o.k group by k) from o

with Correlated scalar subquery must be aggregated to return at most one row.

The aggregate expression is not what makes the subquery scalar. The grouping
is. An empty GROUP BY returns one row, and a GROUP BY on only correlated
columns returns at most one group per set of outer values. The function already
checks the second condition, so the aggregate check is redundant and rejects
valid queries.

What changes are included in this PR?

Removes the aggr_expr.is_empty() check. The GROUP BY check below it is
unchanged, so a GROUP BY on a non-correlated column is still rejected.

These subqueries decorrelate to a plain LEFT JOIN like any other, so no
changes are needed outside the check:

Projection: fact.id, __scalar_sq_1.k AS g
--Left Join: fact.k = __scalar_sq_1.k
----TableScan: fact projection=[id, k]
----SubqueryAlias: __scalar_sq_1
------Aggregate: groupBy=[[dim.k]], aggr=[[]]
--------TableScan: dim projection=[k]

Are these changes tested?

Yes. An existing case in subquery.slt asserted the old error; it now checks
the results and the plan. The negative case for a GROUP BY on a
non-correlated column is unchanged and still passes.

Are there any user-facing changes?

Correlated scalar subqueries that group by only correlated columns and have no
aggregate expression now plan instead of returning an error. No existing plan
changes.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.52%. Comparing base (61bf6b9) to head (778dd5e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24783      +/-   ##
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- Coverage   81.52%   81.52%   -0.01%     
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  Files        1123     1123              
  Lines      405970   406038      +68     
  Branches   405970   406038      +68     
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+ Hits       330978   331026      +48     
- Misses      55627    55645      +18     
- Partials    19365    19367       +2     

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