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-cli/src/eval_file.rs

@@ -39,13 +39,7 @@ pub fn evaluate_file(
     if working_set.find_decl(b"main", &Type::Any).is_some() {
         let args = format!("main {}", args.join(" "));
 
-        if !eval_source(
-            engine_state,
-            stack,
-            &file,
-            &path,
-            PipelineData::new(Span::new(0, 0)),
-        ) {
+        if !eval_source(engine_state, stack, &file, &path, PipelineData::empty()) {
             std::process::exit(1);
         }
         if !eval_source(engine_state, stack, args.as_bytes(), "<commandline>", input) {