🐛 Fixed plaintext field not being cleared when post body is removed

closes https://github.com/TryGhost/Ghost/issues/10557
- add conditional for plaintext changing rather than only being present for when to set a new plaintext value

core/server/models/post.js

@@ -368,7 +368,7 @@ Post = ghostBookshelf.Model.extend({
             // CASE: html is e.g. <p></p>
             // @NOTE: Otherwise we will always update the resource to `plaintext: ''` and Bookshelf thinks that this
             //        value was modified.
-            if (plaintext) {
+            if (plaintext || plaintext !== this.get('plaintext')) {
                 this.set('plaintext', plaintext);
             }
         }

core/test/regression/models/model_posts_spec.js

@@ -507,6 +507,33 @@ describe('Post Model', function () {
                 }).catch(done);
             });
 
+            it('converts html to plaintext if html is null', function (done) {
+                var postId = testUtils.DataGenerator.Content.posts[0].id;
+
+                models.Post.findOne({id: postId}).then(function (results) {
+                    should.exist(results);
+                    results.attributes.html.should.match(/HTML Ipsum Presents/);
+                    should.exist(results.attributes.plaintext);
+                    return models.Post.edit({updated_at: results.attributes.updated_at}, _.extend({}, context, {id: postId}));
+                }).then(function (edited) {
+                    should.exist(edited);
+
+                    edited.attributes.html.should.match(/HTML Ipsum Presents/);
+                    edited.attributes.plaintext.should.match(/HTML Ipsum Presents/);
+                    return edited;
+                }).then(function (edited) {
+                    return models.Post.edit({
+                        mobiledoc: DataGenerator.markdownToMobiledoc(''),
+                        updated_at: edited.attributes.updated_at
+                    }, _.extend({}, context, {id: postId}));
+                }).then(function (edited) {
+                    should.exist(edited);
+                    should.not.exist(edited.attributes.html);
+                    should.not.exist(edited.attributes.plaintext);
+                    done();
+                }).catch(done);
+            });
+
             it('can publish draft post', function (done) {
                 var postId = testUtils.DataGenerator.Content.posts[3].id;