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-engine/src/env.rs

@@ -350,7 +350,7 @@ fn get_converted_value(
                     engine_state,
                     &mut stack,
                     block,
-                    PipelineData::new(val_span),
+                    PipelineData::new_with_metadata(None, val_span),
                     true,
                     true,
                 );

crates/nu-engine/src/eval.rs

@@ -339,10 +339,7 @@ pub fn eval_expression(
         }
         Expr::Call(call) => {
             // FIXME: protect this collect with ctrl-c
-            Ok(
-                eval_call(engine_state, stack, call, PipelineData::new(call.head))?
-                    .into_value(call.head),
-            )
+            Ok(eval_call(engine_state, stack, call, PipelineData::empty())?.into_value(call.head))
         }
         Expr::ExternalCall(head, args, is_subexpression) => {
             let span = head.span;
@@ -352,7 +349,7 @@ pub fn eval_expression(
                 stack,
                 head,
                 args,
-                PipelineData::new(span),
+                PipelineData::empty(),
                 false,
                 false,
                 *is_subexpression,
@@ -536,7 +533,7 @@ pub fn eval_expression(
 
             // FIXME: protect this collect with ctrl-c
             Ok(
-                eval_subexpression(engine_state, stack, block, PipelineData::new(expr.span))?
+                eval_subexpression(engine_state, stack, block, PipelineData::empty())?
                     .into_value(expr.span),
             )
         }
@@ -1087,14 +1084,28 @@ pub fn eval_block(
 
                     match engine_state.find_decl("table".as_bytes(), &[]) {
                         Some(decl_id) => {
-                            let table = engine_state.get_decl(decl_id).run(
-                                engine_state,
-                                stack,
-                                &Call::new(Span::new(0, 0)),
-                                input,
-                            )?;
+                            let table = engine_state.get_decl(decl_id);
 
-                            print_or_return(table, config)?;
+                            if let Some(block_id) = table.get_block_id() {
+                                let block = engine_state.get_block(block_id);
+                                eval_block(
+                                    engine_state,
+                                    stack,
+                                    block,
+                                    input,
+                                    redirect_stdout,
+                                    redirect_stderr,
+                                )?;
+                            } else {
+                                let table = table.run(
+                                    engine_state,
+                                    stack,
+                                    &Call::new(Span::new(0, 0)),
+                                    input,
+                                )?;
+
+                                print_or_return(table, config)?;
+                            }
                         }
                         None => {
                             print_or_return(input, config)?;
@@ -1103,7 +1114,7 @@ pub fn eval_block(
                 }
             }
 
-            input = PipelineData::new(Span::unknown())
+            input = PipelineData::empty()
         }
     }