`HashJoinExec` / `NestedLoopJoinExec` projection `Some(vec![])` becomes `None` after ser/de

[haohuaijin]

Describe the bug

HashJoinExec.projection: Option<Vec<usize>> (and the same field on NestedLoopJoinExec) is serialized as a bare repeated uint32. Proto3 cannot tell None from Some(vec![]), and the decoder treats empty as None:

let projection = if !hashjoin.projection.is_empty() {
    Some(hashjoin.projection.iter().map(|i| *i as usize).collect())
} else {
    None                              // Some(vec![]) folded in here
};

Some(vec![]) means "emit zero columns"; None means "emit the full join schema". The round-trip silently changes the output schema. FilterExec already has a workaround for the same proto3 limitation — these two execs were missed.

To Reproduce

No response

Expected behavior

No response

Additional context

datafusion 53.x, 54.x, and main

[haohuaijin]

take