Cypher: return zero for ungrouped count(*) over empty input #2742

Track the remaining empty-input aggregate behavior separately from the early-WHERE and alias-binding fixes merged in #2502 (3457b21).

For a valid ungrouped query whose MATCH/WHERE produces no input rows, RETURN count(*) AS total should return one result row with total = 0. This follow-up is limited to that aggregate behavior; it does not reopen the resolved alias-binding issue.

Acceptance criteria:

• Add a deterministic regression covering an empty graph and a nonempty graph where WHERE filters out every row.
• Return one row containing integer zero for ungrouped count(*), while preserving nonempty counts and grouped-aggregate behavior.
• Verify the focused Cypher suite.

Thank you to @lorenzozanee for the regression coverage and alias-binding work that helped separate these cases.