chore: Ignore main thread and frame callback return values

We don't do anything with the return value from these callbacks, so it's
OK to ignore them and not convert them to a local handle.

Author: ptomato

test-app/runtime/src/main/cpp/CallbackHandlers.cpp

@@ -704,7 +704,7 @@ int CallbackHandlers::RunOnMainThreadFdCallback(int fd, int events, void *data)
 
     v8::TryCatch tc(isolate);
 
-    if (!cb->Call(context, context->Global(), 0, nullptr).ToLocal(&result)) {}
+    cb->Call(context, context->Global(), 0, nullptr);  // ignore JS return value
 
     cache_.erase(it);
 

test-app/runtime/src/main/cpp/CallbackHandlers.h

@@ -368,9 +368,7 @@ namespace tns {
 
                     v8::TryCatch tc(isolate);
 
-                    if (!cb->Call(context, context->Global(), 1, args).ToLocal(&result)) {
-                        // TODO
-                    }
+                    cb->Call(context, context->Global(), 1, args);  // ignore JS return value
 
                     frameCallbackCache_.erase(entry->id);  // invalidates *entry