Allow legend size to be nil

This updates the legend element to allow the size attribute to be `nil`,
and when `nil` a size class won't be specified for the element. This is
required to support the case where a field has a legend element but
shouldn't have a size class.

There's an example of this kind of component in the GOV.UK Design
System:

https://design-system.service.gov.uk/components/date-input/#if-youre-asking-more-than-one-question-on-the-page

It's currently not possible to re-create this example using purely the
form builder classes as the size is enforced as one of `s`, `m`, `l` or
`xl`.

lib/govuk_design_system_formbuilder/elements/legend.rb

@@ -65,6 +65,8 @@ def retrieve_text(supplied_text)
       end
 
       def size_class(size)
+        return nil if size.nil?
+
         fail "invalid size '#{size}', must be xl, l, m or s" unless size.in?(%w(xl l m s))
 
         %(fieldset__legend--#{size})

spec/govuk_design_system_formbuilder/elements/legend_spec.rb

@@ -0,0 +1,55 @@
+describe GOVUKDesignSystemFormBuilder::Elements::Legend do
+  subject(:html) { described_class.new(builder, object_name, attribute_name, size:).html }
+
+  let(:controller) { ActionController::Base.new }
+  let(:lookup_context) { ActionView::LookupContext.new(nil) }
+  let(:helper) { ActionView::Base.new(lookup_context, {}, controller) }
+  let(:object) { Person.new(name: 'Joey') }
+  let(:object_name) { :person }
+  let(:builder) { GOVUKDesignSystemFormBuilder::FormBuilder.new(object_name, object, helper, {}) }
+  let(:attribute_name) { :name }
+
+  context 'with a nil size' do
+    let(:size) { nil }
+
+    specify 'the size class should be absent' do
+      expect(html).to have_tag('legend', with: { class: 'govuk-fieldset__legend' })
+      expect(html).to_not have_tag('legend', with: { class: 'govuk-fieldset__legend--s' })
+      expect(html).to_not have_tag('legend', with: { class: 'govuk-fieldset__legend--m' })
+      expect(html).to_not have_tag('legend', with: { class: 'govuk-fieldset__legend--l' })
+      expect(html).to_not have_tag('legend', with: { class: 'govuk-fieldset__legend--xl' })
+    end
+  end
+
+  context 'with a "s" size' do
+    let(:size) { 's' }
+
+    specify 'the size class should be present' do
+      expect(html).to have_tag('legend', with: { class: 'govuk-fieldset__legend govuk-fieldset__legend--s' })
+    end
+  end
+
+  context 'with a "m" size' do
+    let(:size) { 'm' }
+
+    specify 'the size class should be present' do
+      expect(html).to have_tag('legend', with: { class: 'govuk-fieldset__legend govuk-fieldset__legend--m' })
+    end
+  end
+
+  context 'with a "l" size' do
+    let(:size) { 'l' }
+
+    specify 'the size class should be present' do
+      expect(html).to have_tag('legend', with: { class: 'govuk-fieldset__legend govuk-fieldset__legend--l' })
+    end
+  end
+
+  context 'with a "xl" size' do
+    let(:size) { 'xl' }
+
+    specify 'the size class should be present' do
+      expect(html).to have_tag('legend', with: { class: 'govuk-fieldset__legend govuk-fieldset__legend--xl' })
+    end
+  end
+end