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Fixes nushell#11711

Previously, syntax `def a [] (echo 4)` was allowed to parse and then
failed with panic duting eval.

Current error:
```
Error: nu::parser::parse_mismatch

  × Parse mismatch during operation.
   ╭─[entry nushell#1:1:1]
 1 │ def a [] (echo 4)
   ·          ────┬───
   ·              ╰── expected definition body closure { ... }
   ╰────
```

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