Clear the scan list when an empty list is received

When the framework receives an empty scan list from supplicant, it does
not clear the local list. So the UI will continue to show the previous
scan list even when there are no APs around. So clear the local list,
when an empty list is received.

CRs-Fixed: 552212
Change-Id: I8715bc31ce0df7bd54c96fcf7d7b73592429f12a