Typo in Slim/App::respond()

[dinacel]

I've didn't review the code, so I don't know if there's side effects:

https://github.com/slimphp/Slim/blob/3.x/Slim/App.php#L389

if (isset($contentLength)) {

is always true, because l.383:

$contentLength = $response->getHeaderLine('Content-Length');

[JoeBengalen]

But getHeaderLine could return empty and the getSize could return null, so isset(null) == false

https://github.com/slimphp/Slim/blob/3.x/Slim/App.php#L383 https://github.com/slimphp/Slim/blob/3.x/Slim/App.php#L385

[dinacel]

My bad, I didn't know that isset(null) == false

[l1905]

isset(null) == false may be unreadable code, why not use is_null(null)?