Fix possible crash when a contact merge results in no UUID.

After merging contacts, it's possible that we don't have a valid
UUID. We need to be careful not to use it.

Kind of a bummer, but the storage sync flow is currently the only flow
where we have this 'possibly not valid UUID'. In the future we should
probably use something else instead of a SignalServiceAddress to keep
that abstraction clean.

libsignal/service/src/main/java/org/whispersystems/signalservice/api/push/SignalServiceAddress.java

@@ -55,6 +55,10 @@ public class SignalServiceAddress {
     return uuid;
   }
 
+  public boolean hasValidUuid() {
+    return !uuid.equals(UuidUtil.UNKNOWN_UUID);
+  }
+
   public String getIdentifier() {
     return uuid.toString();
   }