Fix sending group stories when you're the only group member.

app/src/main/java/org/thoughtcrime/securesms/messages/GroupSendUtil.java

@@ -484,6 +484,7 @@ public final class GroupSendUtil {
                                                        @Nullable CancelationSignal cancelationSignal)
         throws IOException, UntrustedIdentityException
     {
+      // PniSignatures are only needed for 1:1 messages, but some message jobs use the GroupSendUtil methods to send 1:1
       if (targets.size() == 1 && relatedMessageId == null) {
         Recipient            targetRecipient = targetRecipients.get(0);
         SendMessageResult    result          = messageSender.sendDataMessage(targets.get(0), access.get(0), contentHint, message, SignalServiceMessageSender.IndividualSendEvents.EMPTY, urgent, targetRecipient.needsPniSignature());
@@ -689,8 +690,15 @@ public final class GroupSendUtil {
                                                        @Nullable CancelationSignal cancelationSignal)
         throws IOException, UntrustedIdentityException
     {
+      // We only allow legacy sends if you're sending to an empty group and just need to send a sync message.
+      if (targets.isEmpty()) {
+        Log.w(TAG, "Only sending a sync message.");
+        messageSender.sendStorySyncMessage(message, getSentTimestamp(), isRecipientUpdate, manifest);
+        return Collections.emptyList();
+      } else {
         throw new UnsupportedOperationException("Stories can only be send via sender key!");
       }
+    }
 
     @Override
     public @NonNull ContentHint getContentHint() {

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

@@ -268,6 +268,20 @@ public class SignalServiceMessageSender {
     sendGroupMessage(distributionId, recipients, unidentifiedAccess, message.getTimestamp(), content, ContentHint.IMPLICIT, message.getGroupId(), true, SenderKeyGroupEvents.EMPTY, false, false);
   }
 
+  /**
+   * Only sends sync message for a story. Useful if you're sending to a group with no one else in it -- meaning you don't need to send a story, but you do need
+   * to send it to your linked devices.
+   */
+  public void sendStorySyncMessage(SignalServiceStoryMessage message,
+                                   long timestamp,
+                                   boolean isRecipientUpdate,
+                                   Set<SignalServiceStoryMessageRecipient> manifest)
+      throws IOException, UntrustedIdentityException
+  {
+    SignalServiceSyncMessage syncMessage = createSelfSendSyncMessageForStory(message, timestamp, isRecipientUpdate, manifest);
+    sendSyncMessage(syncMessage, Optional.empty());
+  }
+
   /**
    * Send a story using sender key. Note: This is not just for group stories -- it's for any story. Just following the naming convention of making sender key
    * method named "sendGroup*"