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.

src/config_files.rs

@@ -3,7 +3,7 @@ use nu_cli::{eval_config_contents, eval_source, report_error};
 use nu_parser::ParseError;
 use nu_path::canonicalize_with;
 use nu_protocol::engine::{EngineState, Stack, StateWorkingSet};
-use nu_protocol::{PipelineData, Span, Spanned};
+use nu_protocol::{PipelineData, Spanned};
 use nu_utils::{get_default_config, get_default_env};
 use std::fs::File;
 use std::io::Write;
@@ -128,7 +128,7 @@ pub(crate) fn read_default_env_file(engine_state: &mut EngineState, stack: &mut
         stack,
         config_file.as_bytes(),
         "default_env.nu",
-        PipelineData::new(Span::new(0, 0)),
+        PipelineData::empty(),
     );
 
     info!("read_config_file {}:{}:{}", file!(), line!(), column!());
@@ -164,7 +164,7 @@ fn eval_default_config(
         } else {
             "default_config.nu"
         },
-        PipelineData::new(Span::new(0, 0)),
+        PipelineData::empty(),
     );
 
     // Merge the environment in case env vars changed in the config