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-cli/src/config_files.rs

@@ -8,7 +8,7 @@ use nu_path::canonicalize_with;
 use nu_protocol::engine::{EngineState, Stack, StateWorkingSet};
 #[cfg(feature = "plugin")]
 use nu_protocol::Spanned;
-use nu_protocol::{HistoryFileFormat, PipelineData, Span};
+use nu_protocol::{HistoryFileFormat, PipelineData};
 use std::path::PathBuf;
 
 #[cfg(feature = "plugin")]
@@ -38,7 +38,7 @@ pub fn read_plugin_file(
                 stack,
                 &contents,
                 &plugin_filename,
-                PipelineData::new(Span::new(0, 0)),
+                PipelineData::empty(),
             );
         }
     }
@@ -85,7 +85,7 @@ pub fn eval_config_contents(
                 stack,
                 &contents,
                 &config_filename,
-                PipelineData::new(Span::new(0, 0)),
+                PipelineData::empty(),
             );
 
             // Merge the environment in case env vars changed in the config