Fix "Call to undefined function Drupal\system\Plugin\ImageToolkit\imageavif()"

Closes #3526690

core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php

@@ -562,7 +562,7 @@ protected static function checkAvifSupport(): bool {
     }
 
     $tempFile = fopen('php://memory', 'r+');
-    $supported = imageavif(imagecreatetruecolor(1, 1), $tempFile, 0, 10) && fstat($tempFile)['size'] > 0;
+    $supported = function_exists('imageavif') && imageavif(imagecreatetruecolor(1, 1), $tempFile, 0, 10) && fstat($tempFile)['size'] > 0;
     fclose($tempFile);
 
     return $supported;