it('case describing conditionalParamter=NOT NULL'):
    expect(conditionalParamter, 'to be a', 'string') // Guard assertion which can FAIL test. Is it OK? Can I simply **SKIP** assertion WITHOUT test failing
    expect(myStub.args, 'to satisfy', [[ nonConditionalParamter,  ExpressionWhichUsesNonNullConditionalParamterValue ]])

it('case describing conditionalParamter= NULL')
    expect(conditionalParamter, 'to be falsy') // Guard assertion. The same issue
    expect(myStub.args, 'to satisfy', [[ nonConditionalParamter,  ExpressionForNullConditionalParamterValue ]])

class Foo
  def bar(required_param, conditional_param = nil)
    return 'abc' if conditional_param.nil?
    '123'
  end
end

describe Foo do
  subject(:foo) { described_class.new }

  describe '#bar' do
    subject(:bar) { foo.bar(required_param, conditional_param) }

    let(:required_param) { 'required_param' }

    context 'when conditional_param is nil (not present)' do
      let(:conditional_param) { nil }

      it 'returns abc' do
        expect(bar).to eq('abc')
      end
    end

    context 'when conditional_param is not nil' do
      let(:conditional_param) { true }

      it 'returns 123' do
        expect(bar).to eq('123')
      end
    end
  end
end