fix(proto): correctly serialize FilterExec empty projection

[Adez017]

Which issue does this PR close?

• Closes FilterExec empty projection is changed to full projection after serde #21871

Rationale for this change

FilterExec supports two semantically different projection states:

• None → return all columns (full projection)
• Some(vec![]) → return no columns (empty projection)

However, both cases were being serialized identically as an empty vector in the proto representation. During deserialization, an empty vector was always mapped back to None, meaning an empty projection would silently become a full projection after a serde round-trip.

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

No

[askalt]

Currently the code requires a vector allocation to check if the other vector is 0,1,2,3.., but we can do it without allocations using a loop (sample code):

let mut projection = Vec::with_capacity();
let mut is_full_projection =
    filter.projection.len() == input.schema().fields.len();
for (i, idx) in filter.projection.iter().enumerate() {
    let idx = idx as usize;
    is_full_projection &= idx == i;
    projection.push(idx);
}
let projection = if is_full_projection {
    None
} else {
    Some(projection)
};

[Adez017]

Thanks for the feedback @askalt !

[Adez017]

Hi @askalt , i'll be opening a fresh new PR as i am gettign some build error with this branch