[Ruby] Implement new Concept Exercise: nil

[biancapower]

This issue describes how to implement the nil concept exercise for the Ruby track.

Getting started

Please please please read the docs before starting. Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before diving into the implementation, please read up on the following documents:

• The features of v3.
• Rationale for v3.
• What are concept exercise and how they are structured?

Please also watch the following video:

• The Anatomy of a Concept Exercise.

Goal

The goal of this exercise is to teach the student the basics of the Concept of nil in Ruby.

Learning objectives

• Know that nil is a special Ruby object used to represent an "empty" or "default" value
• Know that nil is falsey

Out of scope

• NilClass

Concepts

• nil
• nil? method

Prerequisites

• basics
• booleans

Resources to refer to

Hints

• Data Types in Ruby - True, False, and Nil Explained with Examples

After

• The nil value in Ruby

Representer

This exercise does not require any specific representation logic to be added to the representer.

Analyzer

This exercise does not require any specific analyzer logic to be added to the analyzer.

Implementing

To implement this exercise, please follow these instructions.

Help

If you have any questions while implementing the exercise, please post the questions as comments in this issue.

[neenjaw]

@biancapower do you think I could help with this exercise?

I would like to port over elixir's nil exercise

[biancapower]

@neenjaw please do!

[neenjaw]

See #2046, as it is the follow-up issue.