Body encoding cannot be NIL

# Describe the bug

According to [this comment](https://github.com/pimalaya/himalaya/issues/525#issuecomment-2613920135), maddy seems to send NIL body encoding to FETCH response (within BODYSTRUCTURE) whereas it [cannot be null](https://github.com/pimalaya/himalaya/issues/525#issuecomment-2616931726).

# Steps to reproduce

NC

# Log files

```log
maddy | 2025-01-25T10:30:52.572441139Z [debug] tls.loader.acme/handshake: choosing certificate {"identifier":"imap.services.domain.tld","num_choices":1}
maddy | 2025-01-25T10:30:52.572507024Z [debug] tls.loader.acme/handshake: default certificate selection results {"hash":"ad2fea10f731303445f1be751303e1e2dcfb39760b252
43baa899121e9b6aa02","identifier":"imap.services.domain.tld","issuer_key":"acme-v02.api.letsencrypt.org-directory","managed":true,"subjects":["imap.services.domain.tld"]}
maddy | 2025-01-25T10:30:52.572521371Z [debug] tls.loader.acme/handshake: matched certificate in cache {"expiration":"2025-04-19T14:50:02.000","hash":"ad2fea10f731303445f1be
751303e1e2dcfb39760b25243baa899121e9b6aa02","managed":true,"remote_ip":"192.168.1.2","remote_port":"38888","subjects":["imap.services.domain.tld"]}
maddy | 2025-01-25T10:30:52.575758785Z imap: * OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR CHILDREN UNSELECT MOVE IDLE APPENDLIMIT AUTH=PLAIN COMPRESS] IMAP4rev1 Service Ready
maddy | 2025-01-25T10:30:52.575972430Z imap: 0.0.wnJF090B AUTHENTICATE PLAIN AHByaW50ZXIAZ2MzJTNpRkZGSkR3UmRMNGg0OGI=
maddy | 2025-01-25T10:30:52.803061876Z imap: 0.0.wnJF090B OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR CHILDREN UNSELECT MOVE IDLE APPENDLIMIT I18NLEVEL=1 SORT THREAD=ORDEREDSU
BJECT COMPRESS NAMESPACE] AUTHENTICATE completed
maddy | 2025-01-25T10:30:52.803515145Z imap: 0.1.lgySQXmV SELECT INBOX
maddy | 2025-01-25T10:30:52.803906957Z [debug] imapsql: initialized uidMap for selected mailbox: 2 [8 9]
maddy | 2025-01-25T10:30:52.804090726Z imap: * FLAGS (\Seen \Answered \Flagged \Deleted \Draft old)
maddy | 2025-01-25T10:30:52.804132715Z imap: * OK [PERMANENTFLAGS (\Seen \Answered \Flagged \Deleted \Draft \* old)] Flags permitted.
maddy | 2025-01-25T10:30:52.804191286Z imap: * OK [UNSEEN 2] Message 2 is first unseen
maddy | 2025-01-25T10:30:52.804287068Z imap: * OK [UIDVALIDITY 1437467386] UIDs valid
maddy | 2025-01-25T10:30:52.804301274Z imap: * 2 EXISTS
maddy | 2025-01-25T10:30:52.804324398Z imap: * 0 RECENT
maddy | 2025-01-25T10:30:52.804340288Z imap: * OK [UIDNEXT 10] Predicted next UID
maddy | 2025-01-25T10:30:52.804395293Z imap: 0.1.lgySQXmV OK [READ-WRITE] SELECT completed
maddy | 2025-01-25T10:30:52.804660335Z imap: 0.2.kxqbTv0z FETCH 2:1 (UID FLAGS ENVELOPE BODYSTRUCTURE)
maddy | 2025-01-25T10:30:52.804985040Z [debug] imapsql: resolved false 8:9 to 8:9
maddy | 2025-01-25T10:30:52.805530003Z [debug] imapsql: scanMessages: scanned msgId=8 (seq 1) [UID FLAGS ENVELOPE BODYSTRUCTURE]
maddy | 2025-01-25T10:30:52.805637647Z [debug] imapsql: scanMessages: scanned msgId=9 (seq 2) [UID FLAGS ENVELOPE BODYSTRUCTURE]
maddy | 2025-01-25T10:30:52.805692511Z imap: * 1 FETCH (UID 8 FLAGS (old \Seen) ENVELOPE ("Fri, 24 Jan 2025 18:10:41 +0100" "this is a test" ((NIL NIL "printer" "services.ca
sona.rocks")) ((NIL NIL "printer" "services.domain.tld")) ((NIL NIL "printer" "services.domain.tld")) ((NIL NIL "printer" "services.domain.tld")) NIL NIL NIL "") BODYSTRUCTURE ("text" "plain" ("charset" "us-ascii") NIL NIL NIL 7 1 NIL ("inline" ()) NIL NIL))
maddy | 2025-01-25T10:30:52.805801507Z imap: * 2 FETCH (UID 9 FLAGS (old) ENVELOPE ("Fri, 24 Jan 2025 18:15:31 +0100" "bla" ((NIL NIL "printer" "services.domain.tld")) ((N
IL NIL "printer" "services.domain.tld")) ((NIL NIL "printer" "services.domain.tld")) ((NIL NIL "printer" "services.domain.tld")) NIL NIL NIL "") BODYSTRUCTURE ("text" "plain" ("charset" "us-ascii") NIL NIL NIL 10 1 NIL ("inline" ()) NIL NIL))
maddy | 2025-01-25T10:30:52.805922988Z imap: 0.2.kxqbTv0z OK FETCH completed
```

# Configuration file

NC

# Environment information

NC

---
@petrm feel free to add incomplete or missing information information

Commencez par le traitement IMAP FETCH qui produit BODYSTRUCTURE, en utilisant la requête et la réponse FETCH 2:1 consignées pour reproduire le cas d’un encodage du corps NIL. Suivez la manière dont l’encodage du corps est sélectionné et vérifiez que la réponse respecte l’exigence indiquée dans l’issue selon laquelle elle ne peut pas être NIL.

Rédigé par le modèle d'indexation à partir du texte de l'issue.