Quote empty strings before sending

The empty string is represented in IMAP as a quoted string with zero
characters between double quotes or as a literal with the octet count
of zero. We don't check for it in sendString; sending the empty string
require us to explicitly add quotes. That's unnecessary. The sendString
method should look for empty strings and quote them.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>