Allow empty token usage.

Allow empty token usage.

RFC7252: An empty token value is appropriate e.g., when
no other tokens are in use to a destination, or when requests are
made serially per destination and receive piggybacked responses.

Empty tokens are often used by clients in order to have more compact
messages.

Change-Id: I2b409159a225f06ba80130547ffc9d5cabfee298
Signed-off-by: Stephane Lejeune <stlejeun@cisco.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/3911
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>