Problem/Motivation
People are seeing errors on Taxonomy revision pages when upgrading from 10.2 to 10.3, see #2936995: Add taxonomy term revision UI
This is also being reported in #3317361: New revision fields don't have a default value after making an entity revisionable but that issue seems more general.
IMO we could be more defensive and check for NULL values before calling format() because as we know, Drupal is prone to field values not being there.
Steps to reproduce
From what I've read:
Start on 10.2
Create some terms
Upgrade to 10.3
Visit a term revisions page.
Proposed resolution
Add NULL checks in VersionHistoryController::getRevisionDescription for the revision creation time
Test coverage