Add catch for None type code block in lesson_check

[zkamvar]

There are times when the AST is malformed and does not emit a class for
the code element. We do not want the parser to crash when this happens,
but we also want to notify ourselves that the AST is malformed.

This should not result in an error because as we saw in
#543, the parser itself can
cause these malformations when the lesson itself renders well. Even
though we fixed the previous issue with an updated parser, problems
still persist:
swcarpentry/r-novice-gapminder#696 (comment)

I fully admit that this is a kludge.

This will fix #550

[zkamvar]

Note: when testing on https://github.com/swcarpentry/r-novice-gapminder/tree/a2bf86ecdcb27d5442d8a7366776a50f5b56fad3 with the current branch, I get the following output

./_episodes/01-rstudio-intro.md:152: Unknown or missing code block type r
./_episodes/04-data-structures-part1.md:351: Unknown or missing code block type None
./_episodes/05-data-structures-part2.md:793: Unknown or missing code block type r
./_episodes/05-data-structures-part2.md:803: Unknown or missing code block type r
./_episodes/07-control-flow.md:211: Unknown or missing blockquote type None
./_episodes/07-control-flow.md:225: Unknown or missing blockquote type None
./_episodes/07-control-flow.md:239: Unknown or missing code block type output callout
./_episodes/07-control-flow.md:446: Unknown or missing code block type r
./_episodes/14-tidyr.md:124: Unknown or missing blockquote type None
./_episodes/15-knitr-markdown.md:84: Unknown or missing code block type None
./_episodes/15-knitr-markdown.md:126: Unknown or missing code block type None
./_episodes/15-knitr-markdown.md:134: Unknown or missing code block type None
./_episodes/15-knitr-markdown.md:152: Unknown or missing code block type None
./_episodes/15-knitr-markdown.md:167: Unknown or missing code block type None
./_episodes/15-knitr-markdown.md:190: Unknown or missing code block type None
./_episodes/15-knitr-markdown.md:227: Unknown or missing code block type None
./_extras/guide.md:67: Unknown or missing code block type bash

[maxim-belkin]

Looks good to me! 👍🏻

[maxim-belkin]

Those r and bash blockquotes should be language-r and language-bash, correspondingly.
The Unknown or missing code block type output callout is interesting -- I'd have to look into it locally.

[zkamvar]

Those r and bash blockquotes should be language-r and language-bash, correspondingly.
The Unknown or missing code block type output callout is interesting -- I'd have to look into it locally.

I looked into it and there was a situation where someone added the callout block at the bottom of an R code block, so it is a bona-fide error.

[zkamvar]

Thank you for the quick review @maxim-belkin!