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]!) {
+          // 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 {}
+}

test/omemo_test.dart

@@ -1,5 +1,6 @@
 import 'package:omemo_dart/omemo_dart.dart';
 import 'package:omemo_dart/src/trust/always.dart';
+import 'package:omemo_dart/src/trust/never.dart';
 import 'package:test/test.dart';
 
 void main() {
@@ -336,4 +337,34 @@ void main() {
     );
     expect(messagePlaintext, bobMessage);
   });
+
+  test('Test trust bypassing with empty OMEMO messages', () async {
+    const aliceJid = 'alice@server.example';
+    const bobJid = 'bob@other.server.example';
+      
+    // Alice and Bob generate their sessions
+    final aliceSession = await OmemoSessionManager.generateNewIdentity(
+      aliceJid,
+      NeverTrustingTrustManager(),
+      opkAmount: 1,
+    );
+    final bobSession = await OmemoSessionManager.generateNewIdentity(
+      bobJid,
+      NeverTrustingTrustManager(),
+      opkAmount: 1,
+    );
+
+    // Alice encrypts an empty message for Bob
+    final aliceMessage = await aliceSession.encryptToJid(
+      bobJid,
+      null,
+      newSessions: [
+        await (await bobSession.getDevice()).toBundle(),
+      ],
+    );
+
+    // Despite Alice not trusting Bob's device, we should have encrypted it for his
+    // untrusted device.
+    expect(aliceMessage.encryptedKeys.length, 1);
+  });
 }