api : default signal body to None
api : default signal body to None

Default the signal body to None, so requests without a body don't fail

Change-Id: Ia42d71a338459c8b6543b3d4c5b296d005f4c9ee
Closes-Bug: #1231926

citrix-openstack-build/heat,srznew/heat,takeshineshiro/heat,rdo-management/heat,redhat-openstack/heat,jasondunsmore/heat,dragorosson/heat,citrix-openstack-build/heat,openstack/heat,miguelgrinberg/heat,pratikmallya/heat,takeshineshiro/heat,rickerc/heat_audit,maestro-hybrid-cloud/heat,openstack/heat,rh-s/heat,redhat-openstack/heat,pshchelo/heat,varunarya10/heat,dims/heat,cwolferh/heat-scratch,gonzolino/heat,noironetworks/heat,rh-s/heat,steveb/heat,gonzolino/heat,rickerc/heat_audit,cryptickp/heat,NeCTAR-RC/heat,varunarya10/heat,NeCTAR-RC/heat,steveb/heat,pratikmallya/heat,pshchelo/heat,ntt-sic/heat,rdo-management/heat,ntt-sic/heat,miguelgrinberg/heat,maestro-hybrid-cloud/heat,JioCloud/heat,cwolferh/heat-scratch,noironetworks/heat,srznew/heat,JioCloud/heat,cryptickp/heat,jasondunsmore/heat,dims/heat,dragorosson/heat