Don't warn on format change when campaign content is empty. Closes #634.

frontend/src/components/Editor.vue

@@ -252,6 +252,13 @@ export default {
     },
 
     onFormatChange(format) {
+      if (this.form.body.trim() === '') {
+        this.form.format = format;
+        this.onEditorChange();
+        return;
+      }
+
+      // Content isn't empty. Warn.
       this.$utils.confirm(
         this.$t('campaigns.confirmSwitchFormat'),
         () => {