Bug: Extra null tuples can be emitted by optional match

[royi-luo]

Kùzu version

master

What operating system are you using?

No response

What happened?

See #4240 (comment)

Optional match should output a single null tuple for each probed key that doesn't have a match. However, the current behaviour currently outputs one null tuple for each empty probed vector, which can output more null tuples than are expected. We should fix our left join to align with the expected behaviour.

Are there known steps to reproduce?

No response