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/viewers/griddle.rs

@@ -88,7 +88,7 @@ prints out the list properly."#
                         use_grid_icons,
                     )?)
                 } else {
-                    Ok(PipelineData::new(call.head))
+                    Ok(PipelineData::empty())
                 }
             }
             PipelineData::ListStream(stream, ..) => {
@@ -106,7 +106,7 @@ prints out the list properly."#
                     )?)
                 } else {
                     // dbg!(data);
-                    Ok(PipelineData::new(call.head))
+                    Ok(PipelineData::empty())
                 }
             }
             PipelineData::Value(Value::Record { cols, vals, .. }, ..) => {