Improve empty pipelines (#7383)

# Description

This fix changes pipelines to allow them to actually be empty. Mapping
over empty pipelines gives empty pipelines. Empty pipelines immediately
return `None` when iterated.

This removes a some of where `Span::new(0, 0)` was coming from, though
there are other cases where we still use it.

# User-Facing Changes

None

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-explore/src/nu_common/value.rs

@@ -5,6 +5,7 @@ use super::NuSpan;
 
 pub fn collect_pipeline(input: PipelineData) -> (Vec<String>, Vec<Vec<Value>>) {
     match input {
+        PipelineData::Empty => (vec![], vec![]),
         PipelineData::Value(value, ..) => collect_input(value),
         PipelineData::ListStream(mut stream, ..) => {
             let mut records = vec![];