Eliminate outer joins with empty relations via null-padded projection

[SubhamSinghal]

Which issue does this PR close?

• Closes PropagateEmptyRelation does not eliminate outer joins when one side is empty #21320

Rationale for this change

When one side of a LEFT/RIGHT/FULL outer join is an EmptyRelation, the current PropagateEmptyRelation optimizer rule leaves the join untouched. This means the engine still builds a hash table for the empty side, probes every row from the non-empty side, finds zero matches, and pads NULLs — all wasted work.

The TODO at lines 76-80 of propagate_empty_relation.rs explicitly called out this gap:

// TODO: For LeftOut/Full Join, if the right side is empty, the Join can be eliminated
// with a Projection with left side columns + right side columns replaced with null values.
// For RightOut/Full Join, if the left side is empty, the Join can be eliminated
// with a Projection with right side columns + left side columns replaced with null values.

What changes are included in this PR?

Extends the PropagateEmptyRelation rule to handle 4 previously unoptimized cases by replacing the join with a Projection that null-pads the empty side's columns:

Are these changes tested?

Yes. 4 new unit tests added:

Are there any user-facing changes?

No API changes.