BUG/MINOR: spoe: Don't compare engine name and SPOE scope when both are NULL

BUG/MINOR: spoe: Don't compare engine name and SPOE scope when both are NULL

SPOE filter can be declared without engine name. This is an optional
parameter. But in this case, no scope must be used in the SPOE configuration
file. So engine name and scope are both undefined, and, obviously, we must not
try to compare them.

This patch must be backported in 1.7.