Fix FAQPage schemas with empty answers

Some pages have adjacent h2 tags which means that we will be generating
question and answer pairs that don't actually have an answer.

(An answer in an FAQPage schema is the html that resides after an h2 tag,
the "question" is the text of the h2 tag)

An example page is https://www.gov.uk/check-a-university-is-officially-recognised/listed-bodies
where there is an A-Z list with no content between the V and W h2 titles.

lib/govuk_publishing_components/presenters/machine_readable/faq_page_schema.rb

@@ -24,7 +24,9 @@ def main_entity
       end
 
       def questions_and_answers_markup
-        question_and_answers(page.body).map do |question, value|
+        question_and_answers(page.body)
+          .select { |_, value| value[:answer].present? }
+          .map do |question, value|
           q_and_a_url = section_url(value[:anchor])
 
           {

spec/lib/govuk_publishing_components/presenters/schema_org_spec.rb

@@ -103,7 +103,6 @@
 
         content_item = dragon_guide
 
-
         q_and_a = generate_structured_data(
           content_item: content_item,
           schema: :faq,
@@ -115,6 +114,24 @@
         expect(q_and_a.first["acceptedAnswer"]["text"].strip).to eq("<p>First catch your dragon</p>")
       end
 
+      it "does not include questions when there is no associated answer" do
+        part_body = "<p>First catch your dragon</p>
+                     <h2 id='step-two'>Step two</h2>
+                     <h2 id='step-three'>Step three</h2>
+                     <p>Give it a pat (wear gloves)</p>"
+
+        content_item = dragon_guide
+
+        q_and_a = generate_structured_data(
+          content_item: content_item,
+          schema: :faq,
+          body: part_body
+        ).structured_data['mainEntity']
+
+        expect(q_and_a.count).to eq(2)
+        expect(q_and_a.map { |faq| faq["name"] }).to_not include("Step two")
+      end
+
       it "handles an empty body to ensure that preview works OK" do
         empty_part_body = ""
         content_item = dragon_guide