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/src/shells/enter.rs

@@ -99,7 +99,7 @@ impl Command for Enter {
 
         stack.add_env_var("PWD".into(), new_path);
 
-        Ok(PipelineData::new(call.head))
+        Ok(PipelineData::empty())
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-command/src/shells/exit.rs

@@ -101,7 +101,7 @@ impl Command for Exit {
 
             stack.add_env_var("PWD".into(), new_path);
 
-            Ok(PipelineData::new(call.head))
+            Ok(PipelineData::empty())
         }
     }
 

crates/nu-command/src/shells/mod.rs

@@ -121,7 +121,7 @@ fn switch_shell(
 
     stack.add_env_var("PWD".into(), new_path);
 
-    Ok(PipelineData::new(call.head))
+    Ok(PipelineData::empty())
 }
 
 fn list_shells(