Flags/PermanentFlags somehow contains an empty string

originally reported here: https://github.com/emersion/go-imap/issues/544
tl;dr: go-imap-sql inserts an empty string to Flags, which then gets incorrectly serialized in go-imap for the SELECT response and becomes impossible to get parsed back by go-imap itself

looks like the logic for collecting flags from the mailbox is wrong:
https://github.com/foxcpp/go-imap-sql/blob/c0176dad/mailbox.go#L128-L137

the simplest fix would be
```go
if flag == "" {
continue
}
```

though i'm not sure why it even happens