Check for existing issues

• Completed

Environment

• Linux
• Direct download
• vale version 2.28.3 to 2.29.2

Describe the bug / provide steps to reproduce it

Vale crashes when running it on a document with only a heading following by just a code block. Here is an example document that I used to reproduce the issue (replace <three-backticks> with three actual backticks since gfm doesn't like nesting code blocks):

# A section with only code

``` shell
ls
<three-backticks>

Vale terminates with the following message:

E201 Invalid value [/usr/share/vale/styles/Readability/FleschKincaid.yml:5:1]:

   4  
   5* formula: |
   6    (0.39 * (words / sentences)) + (11.8 * (syllables / words)) - 15.59
   7  

script run: Compile Error: unresolved reference 'words'
        at (main):1:46

Execution stopped with code 1.

This works on versions up to 2.28.2 and stopped working in 2.28.3.