The recent change to the close description fields to handle markdown caused a

regression in the case where the close_description field was null. In this

case, trying to normalize the text would fail, and the API endpoint would

return a 500 with a traceback.
This change makes it so that if a text field is null, we just pass that

through.

Verified that I could once again update un-published review request drafts.
Ran unit tests.