Better handle some elementary edge cases in un/prettify

[rolandwalker]

Description

• sqlglot parse()might return an array of a singleNone`
• don't divide by len() of the current statement if it is zero! Should have been caught.

Checklist

• [x] I've added this contribution to the changelog.md.
• [x] I've added my name to the AUTHORS file (or it's already there).