Refresh the markdown cache if it was `nil`

If the cached html_field for a markdown_field is `nil` while the
mfarkdown_field is not, it needs to be refreshed.

app/models/concerns/cache_markdown_field.rb

@@ -78,6 +78,9 @@ module CacheMarkdownField
   def cached_html_up_to_date?(markdown_field)
     html_field = cached_markdown_fields.html_field(markdown_field)
+    cached = !cached_html_for(markdown_field).nil? && !__send__(markdown_field).nil?
+    return false unless cached
+
     markdown_changed = attribute_changed?(markdown_field) || false
     html_changed = attribute_changed?(html_field) || false

spec/models/concerns/cache_markdown_field_spec.rb

@@ -170,6 +170,12 @@ describe CacheMarkdownField do
       is_expected.to be_truthy
     end
+
+    it 'returns false if the markdown field is set but the html is not' do
+      thing.foo_html = nil
+
+      is_expected.to be_falsy
+    end
   end
   describe '#refresh_markdown_cache!' do