test: Add a test that methods properly return None

Some error in implementation might make methods return something
other than None when they supposed to.

test/test_sdbus_async.py

@@ -219,6 +219,10 @@ async def looong_method(self) -> None:
     def empty_signal(self) -> None:
         raise NotImplementedError
 
+    @dbus_method_async()
+    async def returns_none_method(self) -> None:
+        return
+
 
 class DbusErrorTest(DbusFailedError):
     dbus_error_name = 'org.example.Error'
@@ -310,6 +314,17 @@ async def test_method(self) -> None:
 
         self.assertTrue(await test_object_connection.get_sender())
 
+        with self.subTest("Test method that returns None"):
+            self.assertIsNone(
+
+                await test_object
+                .returns_none_method()  # type: ignore[func-returns-value]
+            )
+            self.assertIsNone(
+                await test_object_connection
+                .returns_none_method()   # type: ignore[func-returns-value]
+            )
+
     async def test_subclass(self) -> None:
         test_object, test_object_connection = initialize_object()