[BUGFIX] Fix array key access in ext_getSetup

[saitho]

What this pr does

Reading nested TypoScript will no longer log a warning when the value can not be accessed. In this case, the second value in the array returned in TypoScript::ext_getSetup will be an empty string. However, the second value is not used by Ext:solr anymore.

This also fixes an issue with PHP 8 where such warning would cause the indexing to fail, when the method is used inside of an indexer.

How to test

Set in TypoScript setup:

plugin.tx_myextension {
  settings {
    foo = bar
  }
}

Place this snipped inside a Solr indexer or Solr indexer hook:

use ApacheSolrForTypo3\Solr\FrontendEnvironment;

// @var $indexItem \ApacheSolrForTypo3\Solr\IndexQueue\Item

$settings =  GeneralUtility::makeInstance(FrontendEnvironment::class)
  ->getConfigurationFromPageId($indexItem->getRootPageUid(), 'plugin.tx_myextension', 0);
$config = $settings->getObjectByPathOrDefault('settings', []);
// $config should return the array ['foo' => 'bar']

Indexing should no longer produce a PHP warning and it should succeed in PHP 8.

Fixes: #3359

[dkd-kaehm]

@saitho
Thanks for that fix.
It will be merged after 11.5 port is created.