Null values Should be Propagated to Upper Levels when NON_NULL Field Returns Null

[ThisaruGuruge]

Description:
According to the [spec](https://spec.graphql.org/June2018/#sec-Errors-and-Non-Nullability, when a field returned null, the GraphQL engine should check the field type whether it is a NON_NULL type or not. If the field type is NON_NULL, the field value cannot be null. In this case, the null value should be propagated to the immediate parent field. If that field type is also NON_NULL, the null value should be further propagated to the next parent object. This should be done until a nullable field is found.
Related #1988