Improve empty pipelines

[sophiajt]

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 after the PR is merged, if necessary. This will help us keep the docs up to date.