reduce: supply <acc> to the closure as pipeline input as well (#13461)

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resolve #13459

# Description
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Make `reduce` supply the accumulator value to closure as pipeline input.

# User-Facing Changes
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Mostly described in #13459

- Should not be a breaking change.
- Allows cleaner patterns like `... | reduce {|it| merge $it}`

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`cargo test --package nu-cli --test main -- commands::reduce` and
`toolkit test stdlib` report no issues

crates/nu-command/src/filters/reduce.rs

@@ -120,8 +120,7 @@ impl Command for Reduce {
             engine_state.signals().check(head)?;
             acc = closure
                 .add_arg(value)
-                .add_arg(acc)
-                .run_with_input(PipelineData::Empty)?
+                .run_with_value(acc)?
                 .into_value(head)?;
         }