Trying to access array offset on value of type null

- BotMan Version: 2.6.1
- PHP Version: 7.4.25
- Messaging Service(s):
- Cache Driver:
- FacebookDriver: 1.11.0

### Description:
Error when calling chatbot with FacebookDriver:

local.ERROR: Trying to access array offset on value of type null {"exception":"[object] (ErrorException(code: 0): Trying to access array offset on value of type null at .......vendor/botman/driver-facebook/src/FacebookDriver.php:516)

So in FacebookDriver:
$facebookResponse->getContent() return nothing
$facebookResponse->getStatusCode() = 500

protected function throwExceptionIfResponseNotOk(Response $facebookResponse)
{
if ($facebookResponse->getStatusCode() !== 200) {
$responseData = json_decode($facebookResponse->getContent(), true);
throw new FacebookException('Error sending payload: '.$responseData['error']['message']);
}
}

### Steps To Reproduce:
Facebook driver tutorial

Start with vendor/botman/driver-facebook/src/FacebookDriver.php around line 516 and inspect throwExceptionIfResponseNotOk. Reproduce the failure using the Facebook driver tutorial, then verify that a non-200 response with empty content no longer triggers an array-offset error and produces an appropriate exception message.

Written by the indexing model from the issue text.