[IOT-1719] Restore empty UUID string conversion

[IOT-1719] Restore empty UUID string conversion

1. Empty strings were being rejected after commit
7c494d7b1a90349f353bf8f3d0f6cd17fe31df14.

2. Add unit test to catch this kind of regression.

3. Improve error logging for this kind of json2cbor failures.

Change-Id: I4608638a5ef952c2949b47ec21e7f610d50f3b82
Signed-off-by: Dan Mihai <Daniel.Mihai@microsoft.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/16019
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Chul Lee <chuls.lee@samsung.com>
Reviewed-by: Greg Zaverucha <gregz@microsoft.com>