Koenig - Fixed empty HTML card rendering undefined

refs https://github.com/TryGhost/Ghost/issues/9623
- add tests for undefined payloads in container cards
- add guard for undefined payload in html card

core/server/lib/mobiledoc/cards/html.js

@@ -2,6 +2,10 @@ module.exports = {
     name: 'html',
     type: 'dom',
     render(opts) {
+        if (!opts.payload.html) {
+            return '';
+        }
+
         // use the SimpleDOM document to create a raw HTML section.
         // avoids parsing/rendering of potentially broken or unsupported HTML
         return opts.env.dom.createRawHTMLSection(opts.payload.html);

core/test/unit/lib/mobiledoc/cards/html_spec.js

@@ -44,4 +44,17 @@ describe('HTML card', function () {
 
         serializer.serialize(card.render(opts)).should.match('<h1>HEADING<');
     });
+
+    it('Renders nothing when payload is undefined', function () {
+        let opts = {
+            env: {
+                dom: new SimpleDom.Document()
+            },
+            payload: {
+                html: undefined
+            }
+        };
+
+        serializer.serialize(card.render(opts)).should.match('');
+    });
 });

core/test/unit/lib/mobiledoc/cards/markdown_spec.js

@@ -72,5 +72,21 @@ describe('Markdown card', function () {
 
             serializer.serialize(card.render(opts)).should.match('<h1 id="heading">HEADING</h1>\n<h2>Heading 2>');
         });
+
+        it('Renders nothing when payload is undefined', function () {
+            let opts = {
+                env: {
+                    dom: new SimpleDom.Document()
+                },
+                payload: {
+                    markdown: undefined
+                },
+                options: {
+                    version: 2
+                }
+            };
+
+            serializer.serialize(card.render(opts)).should.match('');
+        });
     });
 });