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/env/config/config_reset.rs

@@ -108,6 +108,6 @@ impl Command for ConfigReset {
                 }
             }
         }
-        Ok(PipelineData::new(span))
+        Ok(PipelineData::empty())
     }
 }

crates/nu-command/src/env/export_env.rs

@@ -50,7 +50,7 @@ impl Command for ExportEnv {
 
         redirect_env(engine_state, caller_stack, &callee_stack);
 
-        Ok(PipelineData::new(call.head))
+        Ok(PipelineData::empty())
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-command/src/env/let_env.rs

@@ -57,7 +57,7 @@ impl Command for LetEnv {
         } else {
             stack.add_env_var(env_var.item, rhs);
         }
-        Ok(PipelineData::new(call.head))
+        Ok(PipelineData::empty())
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-command/src/env/load_env.rs

@@ -57,7 +57,7 @@ impl Command for LoadEnv {
                         stack.add_env_var(env_var, rhs);
                     }
                 }
-                Ok(PipelineData::new(call.head))
+                Ok(PipelineData::empty())
             }
             None => match input {
                 PipelineData::Value(Value::Record { cols, vals, .. }, ..) => {
@@ -81,7 +81,7 @@ impl Command for LoadEnv {
                             stack.add_env_var(env_var, rhs);
                         }
                     }
-                    Ok(PipelineData::new(call.head))
+                    Ok(PipelineData::empty())
                 }
                 _ => Err(ShellError::UnsupportedInput(
                     "'load-env' expects a single record".into(),