\Drupal\comment\CommentStorage::getMaxThread() and \Drupal\comment\Entity\Comment::getThread() cause deprecations on PHP 8.1

Problem/Motivation

\Drupal\comment\CommentStorage::getMaxThread() can cause deprecations on PHP 8.1 because it returns a NULL and passes it to a string function.
\Drupal\comment\Entity\Comment::getThread() can cause deprecations on PHP 8.1 because it can return a NULL.

Steps to reproduce

Run core/modules/comment/tests/src/Kernel/CommentFieldAccessTest.php on PHP 8.1

Proposed resolution

Maybe we need to adjust \Drupal\comment\CommentStorage::getMaxThreadPerThread() too... or work out a better fix.

Remaining tasks

None

User interface changes

None

API changes

The method Drupal\comment\CommentInterface::getThread() will return the value NULL when the method is called before the comment is saved.

The methods Drupal\comment\CommentStorageInterface::getMaxThread() and Drupal\comment\CommentStorageInterface::getMaxThreadPerThread() will return the value NULL when the commented entity has no comments.

Data model changes

None

Release notes snippet

TBD