Do not attempt to send to unregistered users when using CDS flag.

CDS is slow, and unregistered users will always trigger a CDS lookup on
send (since we can't get their UUID).

This starts skipping sends to unregistered users and shortens the time
window to do a full CDS lookup from every 12 hours to every 6 hours.

libsignal/service/src/main/java/org/whispersystems/signalservice/api/SignalServiceAccountManager.java

@@ -368,6 +368,10 @@ public class SignalServiceAccountManager {
   public Map<String, UUID> getRegisteredUsers(KeyStore iasKeyStore, Set<String> e164numbers, String mrenclave)
       throws IOException, Quote.InvalidQuoteFormatException, UnauthenticatedQuoteException, SignatureException, UnauthenticatedResponseException
   {
+    if (e164numbers.isEmpty()) {
+      return Collections.emptyMap();
+    }
+
     try {
       String                         authorization = this.pushServiceSocket.getContactDiscoveryAuthorization();
       Map<String, RemoteAttestation> attestations  = RemoteAttestationUtil.getAndVerifyMultiRemoteAttestation(pushServiceSocket,