how to test for the default value of [] for `some_array_parameter`?

spec.rb

...
  describe '#some_array_parameter' do
    it 'should default to []' do
      # ???
    end

    context 'the array parameter does something' do
      let(:params) { { :some_array_parameter => [ 'foo', 'bar' ] } }
      ...
    end
  end
...