Missing check for body in truncateWords method

[garak]

It looks like this repository is abandoned, with the last commit 7 years old. Anyway, I see there's some activity in issues/PR, so I'm optimistic.

The problem is just afterhttps://github.com/bluetel/twig-truncate-extension/blob/master/src/TruncateExtension.php#L62 where the value of $body is not checked. Such value can be null, sot the following call new DOMWordsIterator($body) can result into TypeError Argument 1 passed to DOMLettersIterator::__construct() must be an instance of DOMNode, null given

I can provide a PR.