Add additional check in TransportManager

Add additional check to test for empty qualifiedNetworks in
areNetworksValid(). This will avoid edge cases where
TransportManager can get stuck/block all handovers if QNP sends
an empty list followed by valid list with pref nw==current pdn nw

Bug: 146446862
Test: make, manual test
Change-Id: Icfc7169eaa7c5bf2731d9fbeca89384a054c9360