[SPARK-53618][SQL] Turn a FULL JOIN with a false condition into a UNION ALL

[tokoko]

What changes were proposed in this pull request?

PropagateEmptyRelation already rewrites a join whose condition is FalseLiteral for most join types: inner/cross and left semi collapse to an empty relation for example. This PR adds FullOuter handling. With a false condition no row on either side can find a match, so the join is equivalent to a UNION ALL of both sides, each padded with nulls for the other side's columns.

Why are the changes needed?

1 = 0 is not an equi-join condition, so today this plan runs as a BroadcastNestedLoopJoin which is inefficient. After the rewrite both sides are scanned once and fuse into a single whole-stage codegen stage, with no exchange.

Users can usually write the union themselves, but not always. The motivating case is a MERGE with an intentionally non-matching condition, where the join is generated by the MERGE rewrite rather than written by hand:

MERGE INTO target_table AS t
USING source AS s ON 1 = 0
WHEN NOT MATCHED THEN INSERT *
WHEN NOT MATCHED BY SOURCE AND [replaceWhereCondition] THEN DELETE

This is the shape for a replaceWhere-style operation made possible by #52185.

Does this PR introduce any user-facing change?

No. Query results are unchanged; only the optimized plan changes.

How was this patch tested?

New and updated unit tests.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code 2.1.259 (Claude Opus 5)

[HyukjinKwon]

seems like the PR title was truncated. Can you make it complete please?