$initArg<T> function
Used to initialize an Arg with either a default value or a user-provided
value.
If both defaultArg and userArg are null, it returns null.
Implementation
Arg<T>? $initArg<T>(
String name,
Arg<T>? userArg,
Arg<T>? defaultArg,
) {
if (userArg == null && defaultArg == null) return null;
final arg = userArg ?? defaultArg;
return arg!..$generatedName = name;
}