lib: qmi_encdec: Fix encoding a variable length element

If a variable length element has type associated with it and if the length
is specified as zero, encode the element's type and length information in
the QMI message.

Change-Id: Ic4dfe56d0199f5bd622fdd8e18e0d29169cd5bb0
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>