`FilterExec` empty projection is changed to full projection after serde

[askalt]

Consider the following places in the physical plan serde code.

serialization:

datafusion/datafusion/proto/src/physical_plan/mod.rs

Lines 2353 to 2354 in 1897c28

	projection: exec.projection().as_ref().map_or_else(Vec::new, |v| {
	v.iter().map(|x| *x as u32).collect::<Vec<u32>>()

deserealization:

datafusion/datafusion/proto/src/physical_plan/mod.rs

Lines 684 to 694 in 1897c28

	let projection = if !filter.projection.is_empty() {
	Some(
	filter
	.projection
	.iter()
	.map(|i| *i as usize)
	.collect::<Vec<_>>(),
	)
	} else {
	None
	};

If we consider FilterExec with an empty projection (vec![]), then it is serialized as an empty vector. Then the deserialization code makes a FilterExec with None as projection, which semantics is a full projection. The problem is that None and Some(vec![]) have the same proto representation.

The simplest solution:

1. Make (0..fields.len()).collect() vec when None projection is serialized.
2. Make None if this vec is met in the deserialization code.

[Adez017]

take