Do not check whether DoT server list is empty in setDoh

With DDR feature enabled, DoH servers can also be available from
dohParams. setDoh() should not stop if only DoT server list is
empty.

Instead of adding the check for dohParams in the existing if
statement, this change relies on makeDohIdentity() since it can
return a ResultError if both DoT server list and dohParams are
empty.

Note that this change just allows the DnsResolver to start
validating a DoH server when SVCB lookup compltes before DoT
hostname lookup. Before DoT hostname lookup completes and
succeeds, private DNS hostname resolution in NetworkMonitor
is still running.

Bug: 240259333
Test: atest
Change-Id: I16a326df733bb1fa9451a0fabdd9c4d6c002e9e0