[Ruby] Fix ruby client generator to generate setter methods with not null validation

[yujideveloper]

Not null validation was missing for required and non-nullable properties when no other validation was present.
In this changes, it now behaves the same as #valid? and #list_invalid_properties.
And fix an issue where the initialize method did not accept attributes defined in its parent class.

related: #16114, #15895

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@cliffano (2017/07) @zlx (2017/09) @autopp (2019/02)

[wing328]

thanks for the PR

let's give it a try