Handled empty content when sending webmentions

refs https://ghost.slack.com/archives/C02G9E68C/p1676564978732119

- `cheerio` errors when trying to parse `null`

ghost/core/test/e2e-server/services/mentions.test.js

@@ -77,6 +77,17 @@ describe('Mentions Service', function () {
                 assert.equal(mentionMock.isDone(), false);
                 assert.equal(endpointMock.isDone(), false);
             });
+
+            it('Post without content', async function () {
+                let publishedPost = {status: 'published', mobiledoc: markdownToMobiledoc(''), title: 'empty post'};
+                await agent
+                    .post('posts/')
+                    .body({posts: [publishedPost]})
+                    .expectStatus(201);
+
+                assert.equal(mentionMock.isDone(), false);
+                assert.equal(endpointMock.isDone(), false);
+            });
         });
 
         describe(`does send when we expect it to send`, function () {

ghost/webmentions/lib/MentionSendingService.js

@@ -59,13 +59,18 @@ module.exports = class MentionSendingService {
                 // Post should be or should have been published
                 return;
             }
-            await this.#jobService.addJob('sendWebmentions', async () => {
-                await this.sendAll({
-                    url: new URL(this.#getPostUrl(post)),
-                    html: post.get('html'),
-                    previousHtml: post.previous('status') === 'published' ? post.previous('html') : null
+            // make sure we have something to parse before we create a job
+            let html = post.get('html');
+            let previousHtml = post.previous('status') === 'published' ? post.previous('html') : null;
+            if (html || previousHtml) {
+                await this.#jobService.addJob('sendWebmentions', async () => {
+                    await this.sendAll({
+                        url: new URL(this.#getPostUrl(post)),
+                        html: html,
+                        previousHtml: previousHtml
+                    });
                 });
-            });
+            }
         } catch (e) {
             logging.error('Error in webmention sending service post update event handler:');
             logging.error(e);
@@ -104,7 +109,7 @@ module.exports = class MentionSendingService {
      * @param {string|null} [resource.previousHtml]
      */
     async sendAll(resource) {
-        const links = this.getLinks(resource.html);
+        const links = resource.html ? this.getLinks(resource.html) : [];
         if (resource.previousHtml) {
             // We also need to send webmentions for removed links
             const oldLinks = this.getLinks(resource.previousHtml);

ghost/webmentions/test/MentionSendingService.test.js

@@ -188,6 +188,24 @@ describe('MentionSendingService', function () {
             }));
             assert(errorLogStub.calledTwice);
         });
+
+        it('Sends no mentions for posts without html and previous html', async function () {
+            const service = new MentionSendingService({
+                isEnabled: () => true,
+                getPostUrl: () => 'https://site.com/post/',
+                jobService: jobService
+            });
+            const stub = sinon.stub(service, 'sendAll');
+            await service.sendForPost(createModel({
+                status: 'published',
+                html: '',
+                previous: {
+                    status: 'draft',
+                    html: ''
+                }
+            }));
+            assert(stub.notCalled);
+        });
     });
 
     describe('sendAll', function () {
@@ -283,6 +301,16 @@ describe('MentionSendingService', function () {
             assert.strictEqual(scope.isDone(), true);
             assert.equal(counter, 2);
         });
+
+        // cheerio must be served a string
+        it('Does not evaluate links for an empty post', async function () {
+            const service = new MentionSendingService({
+                isEnabled: () => true
+            });
+            const linksStub = sinon.stub(service, 'getLinks');
+            await service.sendAll({html: ``,previousHtml: ``});
+            sinon.assert.notCalled(linksStub);
+        });
     });
 
     describe('getLinks', function () {