Bluetooth: Mesh: HB pub status for unassigned addr

Fixes issue where Heartbeat Publication Status message sends
garbage data when destination field is set to the unassigned address.

MshPRTv1.1, section 4.4.1.2.15:
"When the Destination field is set to the unassigned address, the values
of the CountLog, PeriodLog, TTL, and Features fields shall be set to
0x00 and NetKeyIndex field shall be set to 0x0000."

Signed-off-by: Anders Storrø <anders.storro@nordicsemi.no>