Make code generation respectful to null values

[asvishnyakov]

#1197 fix

Currently if value is null or undefined (checked as value ? doSomething(value) : <any>undefined), it will set undefined to object if value is null, that's incorrect. If we pass null as value, it should still be null value. That's two different values, including API: for example, ASP.NET core will produce 415 error if you will send empty body (undefined) instead of null value.

The easiest way to fix it without making multiple checks for null and undefined is to just pass data value back if it's null or undefined: value ? doSomething(value) : value

[asvishnyakov]

@RicoSuter please review