DPP: Do not reply to PKEX request with identifier if no local identifier

DPP: Do not reply to PKEX request with identifier if no local identifier

The reverse case (local identifier configured but no identifier
received) was already covered, but PKEX is not going to complete
successfully if there is any difference in identifier configuration, so
ignore this other case as well. This avoids unnecessary responses to
PKEX requests with identifier from a device that is ready for PKEX in
general, but not for that particular request.

Signed-off-by: Jouni Malinen <j@w1.fi>