Fix error when pasting HTML that parses to a blank doc

closes bustle#619
- it's possible to paste HTML that parses into a blank doc, if the paste results in a section being replaced then the `post-inserter` would throw an error because it expects the new post to have sections
- adds a guard in the paste event to avoid inserting a blank post

src/js/editor/event-manager.js

@@ -251,10 +251,12 @@ export default class EventManager {
     let targetFormat = this.modifierKeys.shift ? 'text' : 'html';
     let pastedPost = parsePostFromPaste(event, editor, {targetFormat});
 
-    editor.run(postEditor => {
-      let nextPosition = postEditor.insertPost(position, pastedPost);
-      postEditor.setRange(nextPosition);
-    });
+    if (!pastedPost.isBlank) {
+      editor.run(postEditor => {
+        let nextPosition = postEditor.insertPost(position, pastedPost);
+        postEditor.setRange(nextPosition);
+      });
+    }
   }
 
   drop(event) {

tests/acceptance/editor-copy-paste-test.js

@@ -412,3 +412,25 @@ test('paste with shift key pastes plain text', (assert) => {
 
   assert.postIsSimilar(editor.post, expected);
 });
+
+test('paste with html that parses to blank doc doesn\'t error', (assert) => {
+  let expected;
+  let mobiledoc = Helpers.mobiledoc.build(({post, markupSection, marker}) => {
+    expected = post([
+      markupSection('p', [])
+    ]);
+
+    return post([
+      markupSection('p', [marker('abcd')])
+    ]);
+  });
+
+  editor = new Editor({mobiledoc, cards});
+  editor.render(editorElement);
+
+  Helpers.dom.setCopyData('text/html', `<div></div>`);
+  editor.selectRange(editor.post.toRange());
+  Helpers.dom.triggerPasteEvent(editor);
+
+  assert.postIsSimilar(editor.post, expected);
+});