Replace is_resource with different than false

This will avoid an issue when proc_open will return an object instead of
a resource in future PHP versions.

Signed-off-by: Maurício Meneghini Fauth <mauricio@fauth.dev>

libraries/classes/Plugins/Transformations/Abs/ExternalTransformationsPlugin.php

@@ -17,7 +17,6 @@
 use function fgets;
 use function fwrite;
 use function htmlspecialchars;
-use function is_resource;
 use function proc_close;
 use function proc_open;
 use function sprintf;
@@ -140,7 +139,7 @@ public function applyTransformation($buffer, array $options = [], ?FieldMetadata
             ],
         ];
         $process = proc_open($program . ' ' . $options[1], $descriptorspec, $pipes);
-        if (is_resource($process)) {
+        if ($process !== false) {
             fwrite($pipes[0], $buffer);
             fclose($pipes[0]);