add failing tests for stripping null values

spec/jekyll_seo_tag/json_ld_drop_spec.rb

@@ -152,4 +152,12 @@ RSpec.describe Jekyll::SeoTag::JSONLDDrop do
     expect(subject).to have_key("url")
     expect(subject["url"]).to eql("/page.html")
   end
+
+  context "with null values" do
+    let(:metadata) { {} }
+
+    it "does not return null values as json" do
+      expect(subject.to_json).to_not match(/:null/)
+    end
+  end
 end

spec/jekyll_seo_tag_integration_spec.rb

@@ -338,6 +338,10 @@ EOS
       it "minifies JSON-LD" do
         expect(output).to_not match(%r!{.*?\s.*?}!)
       end
+
+      it "removes null values from JSON-LD" do
+        expect(output).to_not match(/:null/)
+      end
     end
   end