Initialize table cell view to empty string

Always initialize the table view cell to the empty string. This removes
the need to set it later in case there's already content there.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Set message and unseen column if value exists

We always set the mailbox messages and unseen column, but some mailboxes
are not selectable. They will be nil in that case or when the values are
unknown. Calling -integerValue will then return zero. We should not set
the value of the text field at all if it's not known, leaving it empty.

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