strip null values from JSON-LD hash

lib/jekyll-seo-tag/json_ld_drop.rb

@@ -73,6 +73,10 @@ module Jekyll
       alias_method :mainEntityOfPage, :main_entity
       private :main_entity
 
+      def to_json
+        to_h.reject { |_k, v| v.nil? }.to_json
+      end
+
       private
 
       attr_reader :page_drop

spec/jekyll_seo_tag/json_ld_drop_spec.rb

@@ -157,7 +157,7 @@ RSpec.describe Jekyll::SeoTag::JSONLDDrop do
     let(:metadata) { {} }
 
     it "does not return null values as json" do
-      expect(subject.to_json).to_not match(/:null/)
+      expect(subject.to_json).to_not match(%r!:null!)
     end
   end
 end

spec/jekyll_seo_tag_integration_spec.rb

@@ -340,7 +340,7 @@ EOS
       end
 
       it "removes null values from JSON-LD" do
-        expect(output).to_not match(/:null/)
+        expect(output).to_not match(%r!:null!)
       end
     end
   end