Fix deprecation warning on PHP 8.1

src/Driver/ParallelFile.php

@@ -141,7 +141,10 @@ final class ParallelFile implements File
 
             try {
                 $data = yield $this->worker->enqueue(new Internal\FileTask('fread', [$length], $this->id));
+
+                if ($data !== null) {
                     $this->position += \strlen($data);
+                }
             } catch (TaskException $exception) {
                 throw new StreamException("Reading from the file failed", 0, $exception);
             } catch (WorkerException $exception) {