char/serial: Fix emptyness handling

The commit 88c1ee73d3231c74ff90bcfc084a7589670ec244
char/serial: Fix emptyness check

Still causes extra NULL byte(s) to be sent.

So if the fifo is empty, do not send an extra NULL byte.

Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Don Slutz <dslutz@verizon.com>
Message-id: 1395160174-16006-1-git-send-email-dslutz@verizon.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
