[Ruby] ruby client build from hash nullable

[tsuwatch]

ref: #4391 #6780

Foo:
  type: object
  properties:
    x:
      type: integer
      nullable: true

On current master:

p OpenapiClient::Foo.new({ x: nil }).to_hash #=> {:x=>nil}
p OpenapiClient::Foo.build_from_hash({ x: nil }).to_hash #=> {}

On this branch:

p OpenapiClient::Foo.build_from_hash({ x: nil }).to_hash #=> {:x=>nil}

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/config/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

@cliffano @zlx @autopp please review

[tsuwatch]

@cliffano @zlx @autopp could you review this if you get a chance?

[autopp]

@tsuwatch Thanks for PR. I'll review it.

[autopp]

@tsuwatch LGTM 👍

[tsuwatch]

@autopp Thakn you!