Add empty implementations of FlutterDesktopMessenger APIs

Author: swift-kim

flutter/shell/platform/tizen/flutter_tizen.cc

@@ -345,25 +345,18 @@ bool FlutterDesktopTextureRegistrarMarkExternalTextureFrameAvailable(
 
 FlutterDesktopMessengerRef FlutterDesktopMessengerAddRef(
     FlutterDesktopMessengerRef messenger) {
-  FT_UNIMPLEMENTED();
   return messenger;
 }
 
-void FlutterDesktopMessengerRelease(FlutterDesktopMessengerRef messenger) {
-  FT_UNIMPLEMENTED();
-}
+void FlutterDesktopMessengerRelease(FlutterDesktopMessengerRef messenger) {}
 
 bool FlutterDesktopMessengerIsAvailable(FlutterDesktopMessengerRef messenger) {
-  FT_UNIMPLEMENTED();
-  return true;
+  return messenger->engine != nullptr;
 }
 
 FlutterDesktopMessengerRef FlutterDesktopMessengerLock(
     FlutterDesktopMessengerRef messenger) {
-  FT_UNIMPLEMENTED();
   return messenger;
 }
 
-void FlutterDesktopMessengerUnlock(FlutterDesktopMessengerRef messenger) {
-  FT_UNIMPLEMENTED();
-}
+void FlutterDesktopMessengerUnlock(FlutterDesktopMessengerRef messenger) {}