[Twisted-Python] PATCH: imap4.py: fix when calling splitQuoted on empty string

To get a list of all folders, Evolution's IMAP client uses this syntax: LIST "" "" This results in an error, since the "" "" part goes through splitQuoted twice (once in IMAP4Server.listWork, the second in IMAP4Server._parseMbox). The second time through the quotes have been removed (leaving an empty string), which causes splitQuoted to return [] instead of ['']. I've attached a simple patch that demonstrates one possible way to fix the problem. Abe