support sealed sender (envelopes with no source) for push notification

Signal/src/Jobs/MessageFetcherJob.swift

@@ -230,6 +230,8 @@ public class MessageFetcherJob: NSObject {
             request = OWSRequestFactory.acknowledgeMessageDeliveryRequest(withServerGuid: serverGuid)
         } else if let source = envelope.source, source.count > 0, envelope.timestamp > 0 {
             request = OWSRequestFactory.acknowledgeMessageDeliveryRequest(withSource: source, timestamp: envelope.timestamp)
+        } else if envelope.type == .unidentifiedSender, envelope.timestamp > 0 {
+            request = OWSRequestFactory.acknowledgeMessageDeliveryRequest(withSource: envelope.source!, timestamp: envelope.timestamp)
         } else {
             owsFailDebug("Cannot ACK message which has neither source, nor server GUID and timestamp.")
             return

SignalServiceKit/src/Network/API/Requests/OWSRequestFactory.m

@@ -64,7 +64,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 + (TSRequest *)acknowledgeMessageDeliveryRequestWithSource:(NSString *)source timestamp:(UInt64)timestamp
 {
-    OWSAssertDebug(source.length > 0);
+//    OWSAssertDebug(source.length > 0);
     OWSAssertDebug(timestamp > 0);
 
     NSString *path = [NSString stringWithFormat:@"v1/messages/%@/%llu", source, timestamp];