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/example_test.rs

@@ -59,7 +59,7 @@ mod test_examples {
         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(Str));
             working_set.add_decl(Box::new(StrJoin));
@@ -215,10 +215,10 @@ mod test_examples {
         );
 
         engine_state
-            .merge_env(&mut stack, &cwd)
+            .merge_env(&mut stack, cwd)
             .expect("Error merging environment");
 
-        let empty_input = PipelineData::new(Span::test_data());
+        let empty_input = PipelineData::empty();
         let result = eval(example.example, empty_input, cwd, engine_state);
 
         // Note. Value implements PartialEq for Bool, Int, Float, String and Block
@@ -320,7 +320,7 @@ mod test_examples {
                 block.pipelines[0].elements.truncate(&n_expressions - 1);
 
                 if !block.pipelines[0].elements.is_empty() {
-                    let empty_input = PipelineData::new(Span::test_data());
+                    let empty_input = PipelineData::empty();
                     Some(eval_block(block, empty_input, cwd, engine_state, delta))
                 } else {
                     Some(Value::nothing(Span::test_data()))