Add test to confirm can receive signals from client with no message bus

lib/src/dbus_client.dart

@@ -132,6 +132,8 @@ class DBusSignalStream extends Stream<DBusSignal> {
         _client._findUniqueName(_rule.sender!);
       }
       _client._addMatch(_rule.toDBusString());
+    } else {
+      _client._connect();
     }
   }
 
@@ -1043,7 +1045,7 @@ class DBusClient {
       }
 
       var signal = DBusSignal(
-          sender: message.sender?.value ?? '',
+          sender: message.sender?.value,
           path: message.path ?? DBusObjectPath.root,
           interface: message.interface?.value ?? '',
           name: message.member?.value ?? '',

lib/src/dbus_server.dart

@@ -385,6 +385,25 @@ class DBusServer {
     }
   }
 
+  /// Emits a signal from the D-Bus server.
+  void emitSignal(
+      {required DBusObjectPath path,
+      required String interface,
+      required String name,
+      Iterable<DBusValue> values = const []}) {
+    var message = DBusMessage(DBusMessageType.signal,
+        flags: {DBusMessageFlag.noReplyExpected},
+        serial: _nextSerial,
+        path: path,
+        interface: DBusInterfaceName(interface),
+        member: DBusMemberName(name),
+        values: values.toList());
+    _nextSerial++;
+    for (var client in _clients) {
+      client.sendMessage(message);
+    }
+  }
+
   /// Listens for connections on a Unix socket.
   Future<DBusAddress> _listenUnixSocket(DBusAddress address) async {
     var path = address.properties['path'];

lib/src/dbus_signal.dart

@@ -3,7 +3,7 @@ import 'dbus_value.dart';
 /// A D-Bus signal.
 class DBusSignal {
   /// Client that sent the signal.
-  final String sender;
+  final String? sender;
 
   /// Path of the object emitting the signal.
   final DBusObjectPath path;

test/dbus_test.dart

@@ -5040,6 +5040,36 @@ void main() {
     expect(node.toXml().toXmlString(), equals('<node/>'));
   });
 
+  test('no message bus - subscribe signal', () async {
+    var server = DBusServer(messageBus: false);
+    var address =
+        await server.listenAddress(DBusAddress.unix(dir: Directory.systemTemp));
+    var client = DBusClient(address, messageBus: false);
+    addTearDown(() async {
+      await client.close();
+      await server.close();
+    });
+
+    var signals =
+        DBusSignalStream(client, interface: 'com.example.Test', name: 'Ping');
+    signals.listen(expectAsync1((signal) {
+      expect(signal.sender, isNull);
+      expect(signal.path, equals(DBusObjectPath('/')));
+      expect(signal.interface, equals('com.example.Test'));
+      expect(signal.name, equals('Ping'));
+      expect(signal.values, equals([DBusString('Hello'), DBusUint32(42)]));
+    }));
+
+    // Ensure client is connected.
+    await client.ping();
+
+    server.emitSignal(
+        path: DBusObjectPath('/'),
+        interface: 'com.example.Test',
+        name: 'Ping',
+        values: [DBusString('Hello'), DBusUint32(42)]);
+  });
+
   test('introspect xml - empty', () {
     expect(() => parseDBusIntrospectXml(''), throwsFormatException);
   });