Check if saved peer config is empty before accessing

If saved peer config is empty, set connection request result should fail
instead of resulting in illegal argument exception.

Also fixing incorrect usage of saved peer config, which cannot be null.

Detailed doc listed in the bug.

Bug: 311064815
Test: atest com.android.server.wifi
Change-Id: I7a4c863fa5336927ab3a2f548c0f485d0c1ae90e
2 files changed