Fuzzed tag values to EntityAutocompleteController::handleAutocomplete can cause deprecation warning

Problem/Motivation

https://www.drupal.org/project/drupal/issues/2829040#comment-14408271

A new one for me:

https://www.drupal.org/pift-ci-job/2316870

Entity.Drupal\KernelTests\Core\Entity\EntityAutocompleteTest
✗
Drupal\KernelTests\Core\Entity\EntityAutocompleteTest
fail: [Other] Line 0 of sites/default/files/simpletest/phpunit-319.xml:
PHPUnit Test failed to complete; Error: PHPUnit 9.5.11 by Sebastian Bergmann and contributors.

Testing Drupal\KernelTests\Core\Entity\EntityAutocompleteTest
.. 2 / 2 (100%)

Time: 00:02.829, Memory: 4.00 MB

OK (2 tests, 14 assertions)

Unsilenced deprecation notices (2)

2x: mb_strtolower(): Passing null to parameter #1 ($string) of type string is deprecated
2x in EntityAutocompleteTest::testSelectionSettingsHandling from Drupal\KernelTests\Core\Entity

Seen here: #3263391: Remove deprecated code from book.module

Steps to reproduce

This seems probably pretty hard to replicate.

Proposed resolution

This actually seems benign and caused by our maybe inadvertent fuzzing since we don't care about the tag value in the first part of the test. The problem would seem to be the interaction of the following code some how.

EntityAutocompleteTest::testSelectionSettingsHandling

    $request->query->set('q', $this->randomString());

EntityAutocompleteController::handleAutocomplete

    if ($input = $request->query->get('q')) {
      $typed_string = Tags::explode($input);
      $typed_string = mb_strtolower(array_pop($typed_string));

array_pop is allowed to return null "If array is empty (or is not an array), null will be returned." Which means, if you give a sufficiently complicated string to the regex in Tags::explode so as to cause it to not match, $typed_string is empty, array_pop return null, and mb_strtolower() complains.

Remaining tasks

Work around an empty explode.

User interface changes

API changes

Data model changes

Release notes snippet