Bug 735200 - [IMAPx] Fails to handle QUOTA response with no quotas

The untagged QUOTA response includes a list containing quota usage and
limits for each quota resource for which a limit applies. Per RFC 2087
section 5.1, "The list contains zero or more triplets.".

Fix the parser to handle an empty list. Additionally, when the IMAP
server supports the quota extension but reports no quotas for a folder,
return G_IO_ERROR_NOT_SUPPORTED rather than G_IO_ERROR_NOT_FOUND, since
Evolution (and maybe other callers) special-cases the former error code.