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/tests/commands/upsert.rs

@@ -43,7 +43,7 @@ fn sets_the_column_from_a_block_full_stream_output() {
     let actual = nu!(
         cwd: "tests/fixtures/formats", pipeline(
         r#"
-            wrap content
+            {content: null}
             | upsert content { open --raw cargo_sample.toml | lines | first 5 }
             | get content.1
             | str contains "nu"
@@ -58,7 +58,7 @@ fn sets_the_column_from_a_subexpression() {
     let actual = nu!(
         cwd: "tests/fixtures/formats", pipeline(
         r#"
-            wrap content
+            {content: null}
             | upsert content (open --raw cargo_sample.toml | lines | first 5)
             | get content.1
             | str contains "nu"