fix: Allow empty OMEMO messages to bypass trust

lib/src/omemo/sessionmanager.dart

@@ -224,8 +224,11 @@ class OmemoSessionManager {
       // We assume that the user already checked if the session exists
       for (final jid in jids) {
         for (final deviceId in _deviceMap[jid]!) {
-          // Only encrypt to devices that are trusted
-          if (!(await _trustManager.isTrusted(jid, deviceId))) continue;
+          // Empty OMEMO messages are allowed to bypass trust
+          if (plaintext != null) {
+            // Only encrypt to devices that are trusted
+            if (!(await _trustManager.isTrusted(jid, deviceId))) continue;
+          }
 
           final ratchetKey = RatchetMapKey(jid, deviceId);
           final ratchet = _ratchetMap[ratchetKey]!;

lib/src/trust/never.dart

@@ -0,0 +1,14 @@
+import 'package:meta/meta.dart';
+import 'package:omemo_dart/src/trust/base.dart';
+
+/// Only use for testing!
+/// An implementation of TrustManager that never trusts any device and thus
+/// has no internal state.
+@visibleForTesting
+class NeverTrustingTrustManager extends TrustManager {
+  @override
+  Future<bool> isTrusted(String jid, int deviceId) async => false;
+
+  @override
+  Future<void> onNewSession(String jid, int deviceId) async {}
+}