'SEARCH UNSEEN' doesn't return unseen messages

# Describe the bug
Sending `SEARCH UNSEEN` to Maddy returns no emails, despite unseen emails being in the mailbox

# Steps to reproduce
- Send emails to a mailbox handled by Maddy's IMAP
- `curl -nv 'imaps://mail.domain.tld/INBOX' -X "SEARCH UNSEEN"`

# Log files
```
< A002 OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR CHILDREN UNSELECT MOVE IDLE APPENDLIMIT I18NLEVEL=1 SORT THREAD=ORDEREDSUBJECT COMPRESS NAMESPACE] AUTHENTICATE completed
> A003 SELECT INBOX
< * FLAGS (\Seen \Answered \Flagged \Deleted \Draft )
< * OK [PERMANENTFLAGS (\Seen \Answered \Flagged \Deleted \Draft \* )] Flags permitted.
< * OK [UNSEEN 5] Message 5 is first unseen
< * 9 EXISTS
< * 0 RECENT
< * OK [UIDNEXT 10] Predicted next UID
< * OK [UIDVALIDITY 1952065945] UIDs valid
< A003 OK [READ-WRITE] SELECT completed
> A004 SEARCH UNSEEN
< * SEARCH
* SEARCH
< A004 OK SEARCH completed
```

```console
$ maddy imap-msgs list mail@domain.tld INBOX
UID 1: Patrycja Rosa - test
[], 2023-06-05 17:27:10 +0200 +0200

UID 2: Patrycja Rosa - test
[], 2023-06-05 17:30:16 +0200 +0200

UID 3: Patrycja Rosa - awoo
[], 2023-06-05 17:32:31 +0200 +0200

UID 4: Patrycja Rosa - awoo
[], 2023-06-05 17:37:26 +0200 +0200

UID 5: Patrycja Rosa - test
[], 2023-06-05 17:37:30 +0200 +0200

UID 6: Patrycja Rosa - test
[], 2023-06-05 17:41:34 +0200 +0200

UID 7: Patrycja Rosa - test
[], 2023-06-05 17:46:05 +0200 +0200

UID 8: Patrycja Rosa - test
[], 2023-06-05 17:50:22 +0200 +0200

UID 9: Patrycja Rosa - test
[], 2023-06-05 17:50:38 +0200 +0200
```

# Environment information

* maddy version: 0.7.0 linux/amd64 go1.20.4