fix: Fix crash when calling getUnacknowledgedRatchets for a new Jid

lib/src/omemo/sessionmanager.dart

@@ -446,18 +446,19 @@ class OmemoSessionManager {
 
   /// Returns the list of device identifiers belonging to [jid] that are yet unacked, i.e.
   /// we have not yet received an empty OMEMO message from.
-  Future<List<int>> getUnacknowledgedRatchets(String jid) async {
+  Future<List<int>?> getUnacknowledgedRatchets(String jid) async {
-    final ret = List<int>.empty(growable: true);
+    return _lock.synchronized(() async {
+      final ret = List<int>.empty(growable: true);
+      final devices = _deviceMap[jid];
+      if (devices == null) return null;
 
-    await _lock.synchronized(() async {
-      final devices = _deviceMap[jid]!;
       for (final device in devices) {
         final ratchet = _ratchetMap[RatchetMapKey(jid, device)]!;
         if (!ratchet.acknowledged) ret.add(device);
       }
-    });
 
-    return ret;
+      return ret;
+    });
   }
 
   /// Mark the ratchet for device [deviceId] from [jid] as acked.

test/omemo_test.dart

@@ -539,7 +539,7 @@ void main() {
     // Alice marks the ratchet as acknowledged
     await aliceSession.ratchetAcknowledged(bobJid, await bobSession.getDeviceId());
     expect(
-      (await aliceSession.getUnacknowledgedRatchets(bobJid)).isEmpty,
+      (await aliceSession.getUnacknowledgedRatchets(bobJid))!.isEmpty,
       true,
     );
   });