eap-authenticator: Assume IKE identity as EAP-Identity if client doesn't send one

eap-authenticator: Assume IKE identity as EAP-Identity if client doesn't send one

Apparently, some clients (e.g. native Android) just send an empty
EAP-Identity response.  We silently ignored that previously and then
used the IKE identity for the actual EAP method.  This change tries to
do something similar (i.e. don't fail if the response is empty), but by
assuming the IKE identity as EAP-Identity, we match that and possibly
can switch configs.

Fixes: 2f2e4abe3c52 ("ikev2: Add support to switch peer configs based on EAP-Identities")