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/misc/history.rs

@@ -54,7 +54,7 @@ impl Command for History {
             if clear {
                 let _ = std::fs::remove_file(history_path);
                 // TODO: FIXME also clear the auxiliary files when using sqlite
-                Ok(PipelineData::new(head))
+                Ok(PipelineData::empty())
             } else {
                 let history_reader: Option<Box<dyn ReedlineHistory>> =
                     match engine_state.config.history_file_format {