\Drupal\KernelTests\KernelTestBase::bootKernel() causing deprecations in PHP 8.1

Problem/Motivation

This is blocker to run a lot of tests without deprecation warnings

Steps to reproduce

https://3v4l.org/0rAA5

$v=NULL;var_dump(strpos($v, 's'));

Run \Drupal\Tests\contact\Kernel\MessageEntityTest

vendor/bin/phpunit -c core/phpunit.xml.dist --colors=always --debug -v \
core/modules/contact/tests/src/Kernel/MessageEntityTest.php
PHPUnit 8.5.21 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.1.0RC3
Configuration: /var/www/html/web/core/phpunit.xml.dist

Testing Drupal\Tests\contact\Kernel\MessageEntityTest
Test 'Drupal\Tests\contact\Kernel\MessageEntityTest::testMessageMethods' started
Test 'Drupal\Tests\contact\Kernel\MessageEntityTest::testMessageMethods' ended


Time: 1.73 seconds, Memory: 6.00 MB

OK (1 test, 25 assertions)

Unsilenced deprecation notices (1)

  1x: strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated
    1x in MessageEntityTest::testMessageMethods from Drupal\Tests\contact\Kernel

Proposed resolution

Use empty string instead of NULL for $namespace and $autoload

Remaining tasks

review/commit

User interface changes

API changes

Data model changes

Release notes snippet