[BUGFIX] Handle absolute web paths in FormEngineUtility::getIconHtml

Since the introduction of always absolute (web) paths inside the TYPO3
backend, the TCA ctrl option `selicon_field` didn't work anymore since
relative web paths like `fileadmin/file.png` changed to
`/fileadmin/file.png`, which were not handled and resulted in a missing
icon.

This patch extends the condition for checking for absolute paths and
prevents the GU:getFileAbsFileName method from turning web paths into
empty strings.

Resolves: #95572
Resolves: #97442
Related: #95027
Releases: main, 11.5
Change-Id: I1b82f71fe4f0636bd786d102f361ca60c1294af9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77431


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>

typo3/sysext/backend/Classes/Form/Utility/FormEngineUtility.php

@@ -17,6 +17,7 @@ namespace TYPO3\CMS\Backend\Form\Utility;
 
 use TYPO3\CMS\Backend\Utility\BackendUtility;
 use TYPO3\CMS\Core\Cache\CacheManager;
+use TYPO3\CMS\Core\Core\Environment;
 use TYPO3\CMS\Core\Imaging\Icon;
 use TYPO3\CMS\Core\Imaging\IconFactory;
 use TYPO3\CMS\Core\Utility\ArrayUtility;
@@ -122,8 +123,12 @@ class FormEngineUtility
     public static function getIconHtml($icon, $alt = '', $title = '')
     {
         $icon = (string)$icon;
-        $absoluteFilePath = GeneralUtility::getFileAbsFileName($icon);
-        if (!empty($absoluteFilePath) && is_file($absoluteFilePath)) {
+        if (PathUtility::isAbsolutePath($icon)) {
+            $absoluteFilePath = $icon;
+        } else {
+            $absoluteFilePath = GeneralUtility::getFileAbsFileName($icon);
+        }
+        if (!empty($absoluteFilePath) && (is_file($absoluteFilePath)) || is_file(Environment::getPublicPath() . $absoluteFilePath)) {
             return '<img'
                 . ' loading="lazy" '
                 . ' src="' . htmlspecialchars(PathUtility::getAbsoluteWebPath($absoluteFilePath)) . '"'