Fix of COUNT(*) returning NULL from an empty group by

[Shmuma]

Closes #292
Closes #297

[redcatbear]

Engine team gave us the tip to also check for HAVING clauses.

if (hasSingleGroupAggregation(select) && !hasAggregateFunction(selectList) &&  !hasAggregateFunction(having) {

Also, they pointed out that in a visitor getChildren should not be necessary. @Shmuma, contact @narmion for context please.

[Shmuma]

Engine team gave us the tip to also check for HAVING clauses.

if (hasSingleGroupAggregation(select) && !hasAggregateFunction(selectList) &&  !hasAggregateFunction(having) {

Also, they pointed out that in a visitor getChildren should not be necessary. @Shmuma, contact @narmion for context please.

Regarding HAVING is a good point, will check.
New method getChildren is absolutely necessary, without it, it is not possible to traverse the tree properly.

[sonarqubecloud]

Quality Gate failed

Failed conditions
26.8% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud