Fix the potential NPE when starting provisioning with FILS.

Starting provisioning with enabled FILS is unlike the normal case
that always happens after L2 connection has been established, at
this point we already know the current bssid. However, for FILS
connection, the current bssid is still null when starting prov.

Bug: 155696520
Test: atest NetworkStackTests NetworkStackIntegrationTests
Change-Id: I3c29c6ad95c23cea2b3f374fb9dc35a8f54a6427