Handle undefined quotalist when set quota on some imap server (e.g. cyrus)

Some imap server doesn't return a QUOTA response after a SETQUOTA request, which makes the quotalist variable undefined. The following session is on cyrus imap server:
```
A1 getquota user/test
* QUOTA user/test (STORAGE 12 56320)
A1 OK Completed
A2 setquota user/test (STORAGE 56444)
A2 OK Completed
A3 getquota user/test
* QUOTA user/test (STORAGE 12 56444)
A3 OK Completed
```