[BUGFIX] follow-up: false as field value is not allowed

This change is a follow-up of #3901, which was introduced to allow following values in Apache Solr fields:
* numeric 0
* boolean false
which lead to countless-unwanted numeric fields with 0 value in Apache Solr documents.
The background: TypoScript can return the strings only, and all empty values lead to numeric 0.

This change tries to avoid mispelled casting of numbers and lets the 
`AbstractIndexer::ensureFieldValueType()` return PHPs `null` if the value isn't a number. 

Relates: #3900, #3901

Classes/IndexQueue/AbstractIndexer.php

@@ -24,6 +24,7 @@
 use TYPO3\CMS\Core\Core\Environment;
 use TYPO3\CMS\Core\TypoScript\Parser\TypoScriptParser;
 use TYPO3\CMS\Core\Utility\GeneralUtility;
+use TYPO3\CMS\Core\Utility\MathUtility;
 use TYPO3\CMS\Frontend\Controller\TypoScriptFrontendController;
 use UnexpectedValueException;
 
@@ -282,12 +283,12 @@ protected function ensureFieldValueType(mixed $value, string $fieldType): mixed
         switch ($fieldType) {
             case 'int':
             case 'tInt':
-                $value = (int)$value;
+                $value = MathUtility::canBeInterpretedAsInteger($value) ? (int)$value : null;
                 break;
 
             case 'float':
             case 'tFloat':
-                $value = (float)$value;
+                $value = MathUtility::canBeInterpretedAsInteger($value) || MathUtility::canBeInterpretedAsFloat($value) ? (float)$value : null;
                 break;
             case 'long':
                 // long and double do not exist in PHP
@@ -297,15 +298,15 @@ protected function ensureFieldValueType(mixed $value, string $fieldType): mixed
                 // remove anything that's not a number or negative/minus sign
                 $value = preg_replace('/[^0-9\\-]/', '', $value);
                 if (trim($value) === '') {
-                    $value = 0;
+                    $value = null;
                 }
                 break;
             case 'double':
             case 'tDouble':
             case 'tDouble4':
                 // as long as it's numeric we'll take it, int or float doesn't matter
                 if (!is_numeric($value)) {
-                    $value = 0;
+                    $value = null;
                 }
                 break;