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 <66c835befd325c6202670128661db156a1000618@cisco.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/3911
Tested-by: jenkins-iotivity <09cb29e8a2b473a2c978382eec13ee06fa017bda@opendaylight.org>
Reviewed-by: Jon A. Cruz <d4ae605477945a543a3ecda8955c93978db16a84@osg.samsung.com>

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