person_nil_to_s_override.rb

class NilClass
  def to_s
    nil
  end
end

class Person
  def initialize(options)
    @first_name = options[:first_name]
    @middle_name = options[:middle_name]
    @last_name = options[:last_name]
  end

  def full_name
    "#{@first_name} #{@middle_name} #{@last_name}"
  end
end

person = Person.new(first_name: 'Kori', last_name: 'Roys')
puts person.full_name # => Kori #<NilClass:0x00000000000004> Roys

person_normal.rb

class Person
  def initialize(options)
    @first_name = options[:first_name]
    @middle_name = options[:middle_name]
    @last_name = options[:last_name]
  end

  def full_name
    "#{@first_name} #{@middle_name} #{@last_name}"
  end
end

person = Person.new(first_name: 'Kori', last_name: 'Roys')
puts person.full_name # => Kori  Roys