Prevent fatal error if revision belongs to anonymous user.

Problem/Motivation

Need diff to work if a revision belongs to the anonymous author.

This appears to be related to an issue that was fixed at some point in the 2.x branch (see the closed issue https://www.drupal.org/project/diff/issues/3088274).

Steps to reproduce

Change owner of a node revision to the anonymous user (UID=0), then try to make a diff.
The error that returns is Error: Call to a member function getDisplayName() on null in Drupal\diff\DiffLayoutBase->buildRevisionData() (line 189 of /opt/drupal/web/modules/contrib/diff/src/DiffLayoutBase.php).

In my case, the user who had created the node was deleted and its content assigned to the anonymous user. Another user then edited the node. When trying to diff the two I get the error.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes