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-command/src/dataframe/test_dataframe.rs

@@ -22,7 +22,7 @@ pub fn test_dataframe(cmds: Vec<Box<dyn Command + 'static>>) {
     let delta = {
         // Base functions that are needed for testing
         // Try to keep this working set small to keep tests running as fast as possible
-        let mut working_set = StateWorkingSet::new(&*engine_state);
+        let mut working_set = StateWorkingSet::new(&engine_state);
         working_set.add_decl(Box::new(Let));
         working_set.add_decl(Box::new(ToDataFrame));
         working_set.add_decl(Box::new(ToLazyFrame));
@@ -49,7 +49,7 @@ pub fn test_dataframe(cmds: Vec<Box<dyn Command + 'static>>) {
         let start = std::time::Instant::now();
 
         let (block, delta) = {
-            let mut working_set = StateWorkingSet::new(&*engine_state);
+            let mut working_set = StateWorkingSet::new(&engine_state);
             let (output, err) = parse(
                 &mut working_set,
                 None,
@@ -75,7 +75,7 @@ pub fn test_dataframe(cmds: Vec<Box<dyn Command + 'static>>) {
             &engine_state,
             &mut stack,
             &block,
-            PipelineData::new(Span::test_data()),
+            PipelineData::empty(),
             true,
             true,
         ) {