fix optional type set before get

[mlch911]

When we have a optional type var with default value. We may set it before call get. So the value will change to the default Value.

@AssociatedObject(.retain(.atomic))
var string: String? = "text"

string = "abc"
print(string) // => text ❌

[p-x9]

Thank you.
I have made one comment on the improvement suggestion, please check it out.

[p-x9]

Thanks for the fix.