fix(postgres): allow-list nullable-side cols at outer-join boundaries

Old pass marked anything in a join's Output that wasn't a pass-through
from the preserved-side child. That misclassified sibling `(SubPlan N)`
refs as nullable — they're computed independently of the join's NULL
extension.

Now: at each outer-join, collect qualified col refs (`alias.col`) from
the nullable-side subtree's leaves (skipping SubPlan/InitPlan branches)
and mark only outputs that mention one. Pass-throughs and computed exprs
over nullable-side cols stay covered; subplan/param/constant refs stay
unmarked.

#4285 (comment)

Author: luca992