'Empty' exercise chunks aren't treated as exercises

[cpsievert]

Running the tutorial below yields a misleading error of:

Error in is_code_identical(user, solution) : 
  No solution is provided for this exercise.

This is because the foo chunk isn't being treated as an exercise, so the foo-check is being evaluated outside of a grade_learnr() context. Adding an empty line to the foo chunk fixes the problem, but either a more informative error should be thrown, or it should "just work"

---
title: "Issue #410"
output: learnr::tutorial
runtime: shiny_prerendered
---

```{r setup, include=FALSE}
library(learnr)
library(gradethis)
tutorial_options(exercise.checker = grade_learnr)
knitr::opts_chunk$set(echo = FALSE)
```

## Minimal example

This chunk isn't treated as an exercise!

```{r foo, exercise = TRUE}
```

```{r foo-solution}
sum(1)
```

```{r foo-check}
grade_code()
```

[KatherineCox]

Is this related to #172 ?