Review Board < 2.0.7 breaks when trying to render changes to the Note

field if adding content for the first time after the review request has

already been published. The old value for the field was 'None', which

broke the rendering due to the expectation of strings.
While Review Board 2.0.7 is getting a fix for this, we still want to

work around it for earlier releases. This adds a new

render_change_entry_html() function that just normalizes the values

before calling the parent function, ensuring we're dealing with empty

strings.

Tested without the fix in /r/6290/ and saw that the change could properly

render. Previously, the page was showing a traceback.