Restrict match conditions of TelephonyNetworkSpecifier#canBeSatisfied

TelephonyNetworkSpecifier will now treat null as matching nothing. When
the request specifies a TelephonyNetworkSpecifier while the network does
not, this should not be treated as a match.

Bug: 154703135
Test: atest android.net.TelephonyNetworkSpecifierTest
Change-Id: I329110e929995c9eae6c6ce33b5414777acea1e1