The second section in a document without an exercise cannot be skipped

[robbyshaver]

The issue is that the method for communicating "skip" from tutorial-format.js to the tutor code uses the data-label attribute of the exercise. If there is no exercise, the label is "undefined", which works for the first exercise-less section, but fails on the second. Hmmm ....

Sample code:

---
title: "Two exercise-less sections"
output: 
  tutor::tutorial:
    progressive: true
    allow_skip: true
runtime: shiny_prerendered
---

```{r setup, include=FALSE}
library(tutor)
```

## Welcome

### Section 1

This section can be skipped.

### Section 2

But not this one

### Section 3

Sad! Write the R code required to add two plus two:

```{r two-plus-two, exercise=TRUE}

```