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/prompt_update.rs

@@ -4,7 +4,7 @@ use log::info;
 use nu_engine::eval_subexpression;
 use nu_protocol::{
     engine::{EngineState, Stack, StateWorkingSet},
-    Config, PipelineData, Span, Value,
+    Config, PipelineData, Value,
 };
 use reedline::Prompt;
 
@@ -37,12 +37,8 @@ fn get_prompt_string(
                 let block = engine_state.get_block(block_id);
                 let mut stack = stack.captures_to_stack(&captures);
                 // Use eval_subexpression to force a redirection of output, so we can use everything in prompt
-                let ret_val = eval_subexpression(
-                    engine_state,
-                    &mut stack,
-                    block,
-                    PipelineData::new(Span::new(0, 0)), // Don't try this at home, 0 span is ignored
-                );
+                let ret_val =
+                    eval_subexpression(engine_state, &mut stack, block, PipelineData::empty());
                 info!(
                     "get_prompt_string (block) {}:{}:{}",
                     file!(),
@@ -62,12 +58,7 @@ fn get_prompt_string(
             Value::Block { val: block_id, .. } => {
                 let block = engine_state.get_block(block_id);
                 // Use eval_subexpression to force a redirection of output, so we can use everything in prompt
-                let ret_val = eval_subexpression(
-                    engine_state,
-                    stack,
-                    block,
-                    PipelineData::new(Span::new(0, 0)), // Don't try this at home, 0 span is ignored
-                );
+                let ret_val = eval_subexpression(engine_state, stack, block, PipelineData::empty());
                 info!(
                     "get_prompt_string (block) {}:{}:{}",
                     file!(),