FIX: when quoting add a newline

so **highlight** > quote

Does not end like:

[quote]
test
[quote]~

app/assets/javascripts/discourse/components/d-editor.js.es6

@@ -589,9 +589,10 @@ export default Ember.Component.extend({
 
     if (post.length > 0) {
       post = post.replace(/^\n*/, "\n\n");
+    } else {
+      post = "\n";
     }
 
-
     const value = pre + text + post;
     const $textarea = this.$('textarea.d-editor-input');