Interoperability: Mail.ru FETCH parsing failures (Himalaya / real IMAP)

## Summary

Mail.ru (and clients built on **imap-codec**, e.g. Himalaya via **imap-next** / **email-lib**) can fail to parse `FETCH` responses: the codec reports parse errors and downstream may skip entire messages, which shows up as an empty or incomplete envelope list even though the server has mail.

## What we observed

1. **Empty quoted string in addresses** — Envelope address lists use `("" NIL "user" "domain")` for an empty display name. RFC 3501 allows `quoted` to be empty (`DQUOTE DQUOTE`), but `imap-codec`’s `quoted()` parser did not accept zero characters between quotes (failure inside `nom::escaped` / `take_while1`).

2. **`body-fld-param` with `NIL` value** — Extension lists such as `("boundary" NIL)` appear in `BODYSTRUCTURE`. Strict RFC expects `string SP string` pairs; some servers send `NIL` for the value. Parsing fails without a quirk (similar spirit to existing `quirk_body_fld_enc_nil_to_empty`).

3. **Fixture / nesting** — A real line uses **`multipart/mixed`** whose first part is **`multipart/alternative`**, so the `BODYSTRUCTURE` argument correctly begins with three `(` after the keyword. Tests should reflect that shape so the full `FETCH` line parses end-to-end.

## Proposed direction

- Fix **`quoted()`** to accept `""` (minimal, spec-aligned).
- Add a **default `quirk`** feature (e.g. `quirk_body_fld_param_nil_value`) to treat `NIL` as an empty string for the second field in `body-fld-param` pairs when inside `(...)`.
- Add **regression tests**: minimal `FETCH` line for `""` in addresses; optional integration test with a captured `BODYSTRUCTURE` line; targeted parsers for envelope vs body if useful for future bisection.

Happy to open a PR with this split (strict fix vs quirk) if that matches how you want to track server deviations.