[CDF-441] - Fixing previous solution

It's ok, like it is, but it's safer and more idiomatic to write the strictly equivalent `async == null`.
I'm of the opinion of using `undefined` only with the identity operator: `===`.
Some don't even like using `undefined`, because it can be inadvertently changed...
Nowadays, JavaScript code compressors and rewriters translate `undefined` to the equivalent, safer and smaller expression: `void 0`.
I restrict myself to using the cumbersome `typeof foo === 'undefined'` idiom when it is possible that the `foo` identifier might not be declared/defined — its _bare_ use would throw an exception. I never use it to test if the value of a surely declared identifier is `undefined`.