[Linux] Allow BasicMessageChannel sending and responding to null message (flutter/engine#42808)

## Description

This PR changes how a null message is handled by the Linux engine for a basic message channel.

**Before**:

- when receiving a null message a warning was emitted.
- `fl_binary_messenger_send_response` was called but failed and the application exited.

```
** (bug:9866): WARNING **: 23:13:42.109: Failed to decode message: Unexpected end of data

** (bug:9866): CRITICAL **: 23:13:42.109: gboolean send_response(FlBinaryMessenger *, FlBinaryMessengerResponseHandle *, GBytes *, GError **): assertion 'response_handle->response_handle != nullptr' failed
```

**After**:

- Receiving a null message is handled as expected from the framework side documentation:

9287e81d52/packages/flutter/lib/src/services/platform_channel.dart (L149-L150)

## Related Issue

Fixes https://github.com/flutter/flutter/issues/128704

## Tests

Adds 2 tests.

engine/src/flutter/shell/platform/linux/fl_basic_message_channel.cc

@@ -232,7 +232,6 @@ G_MODULE_EXPORT void fl_basic_message_channel_send(FlBasicMessageChannel* self,
                                                    GAsyncReadyCallback callback,
                                                    gpointer user_data) {
   g_return_if_fail(FL_IS_BASIC_MESSAGE_CHANNEL(self));
-  g_return_if_fail(message != nullptr);
 
   g_autoptr(GTask) task =
       callback != nullptr ? g_task_new(self, cancellable, callback, user_data)

engine/src/flutter/shell/platform/linux/fl_basic_message_channel_test.cc

@@ -62,7 +62,8 @@ TEST(FlBasicMessageChannelTest, SendMessageWithoutResponse) {
 }
 // NOLINTEND(clang-analyzer-core.StackAddressEscape)
 
-// Called when the message response is received in the SendMessage test.
+// Called when the message response is received in the SendMessageWithResponse
+// test.
 static void echo_response_cb(GObject* object,
                              GAsyncResult* result,
                              gpointer user_data) {
@@ -189,3 +190,35 @@ TEST(FlBasicMessageChannelTest, ReceiveMessage) {
   // Blocks here until response_cb is called.
   g_main_loop_run(loop);
 }
+
+// Called when the message response is received in the
+// SendNullMessageWithResponse test.
+static void null_message_response_cb(GObject* object,
+                                     GAsyncResult* result,
+                                     gpointer user_data) {
+  g_autoptr(GError) error = nullptr;
+  g_autoptr(FlValue) message = fl_basic_message_channel_send_finish(
+      FL_BASIC_MESSAGE_CHANNEL(object), result, &error);
+  EXPECT_NE(message, nullptr);
+  EXPECT_EQ(error, nullptr);
+
+  EXPECT_EQ(fl_value_get_type(message), FL_VALUE_TYPE_NULL);
+
+  g_main_loop_quit(static_cast<GMainLoop*>(user_data));
+}
+
+// Checks sending a null message with a response works.
+TEST(FlBasicMessageChannelTest, SendNullMessageWithResponse) {
+  g_autoptr(GMainLoop) loop = g_main_loop_new(nullptr, 0);
+
+  g_autoptr(FlEngine) engine = make_mock_engine();
+  FlBinaryMessenger* messenger = fl_binary_messenger_new(engine);
+  g_autoptr(FlStandardMessageCodec) codec = fl_standard_message_codec_new();
+  g_autoptr(FlBasicMessageChannel) channel = fl_basic_message_channel_new(
+      messenger, "test/echo", FL_MESSAGE_CODEC(codec));
+  fl_basic_message_channel_send(channel, nullptr, nullptr,
+                                null_message_response_cb, loop);
+
+  // Blocks here until null_message_response_cb is called.
+  g_main_loop_run(loop);
+}

engine/src/flutter/shell/platform/linux/fl_standard_message_codec.cc

@@ -420,6 +420,10 @@ static GBytes* fl_standard_message_codec_encode_message(FlMessageCodec* codec,
 static FlValue* fl_standard_message_codec_decode_message(FlMessageCodec* codec,
                                                          GBytes* message,
                                                          GError** error) {
+  if (g_bytes_get_size(message) == 0) {
+    return fl_value_new_null();
+  }
+
   FlStandardMessageCodec* self =
       reinterpret_cast<FlStandardMessageCodec*>(codec);
 

engine/src/flutter/shell/platform/linux/fl_standard_message_codec_test.cc

@@ -60,6 +60,19 @@ TEST(FlStandardMessageCodecTest, EncodeNull) {
   EXPECT_STREQ(hex_string, "00");
 }
 
+TEST(FlStandardMessageCodecTest, DecodeNull) {
+  // Regression test for https://github.com/flutter/flutter/issues/128704.
+
+  g_autoptr(FlStandardMessageCodec) codec = fl_standard_message_codec_new();
+  g_autoptr(GBytes) data = g_bytes_new(nullptr, 0);
+  g_autoptr(GError) error = nullptr;
+  g_autoptr(FlValue) value =
+      fl_message_codec_decode_message(FL_MESSAGE_CODEC(codec), data, &error);
+
+  EXPECT_FALSE(value == nullptr);
+  EXPECT_EQ(fl_value_get_type(value), FL_VALUE_TYPE_NULL);
+}
+
 static gchar* encode_bool(gboolean value) {
   g_autoptr(FlValue) v = fl_value_new_bool(value);
   return encode_message(v);

engine/src/flutter/shell/platform/linux/public/flutter_linux/fl_basic_message_channel.h

@@ -171,7 +171,8 @@ gboolean fl_basic_message_channel_respond(
 /**
  * fl_basic_message_channel_send:
  * @channel: an #FlBasicMessageChannel.
- * @message: message to send, must match what the #FlMessageCodec supports.
+ * @message: (allow-none): message to send, must match what the #FlMessageCodec
+ * supports.
  * @cancellable: (allow-none): a #GCancellable or %NULL.
  * @callback: (scope async): (allow-none): a #GAsyncReadyCallback to call when
  * the request is satisfied or %NULL to ignore the response.