FIX check if there is a buffer before cleaning it.

[JayyWalker]

I was getting the following error

{
    "statusCode": 500,
    "error": {
        "type": "SERVER_ERROR",
        "description": "ERROR: ob_clean(): failed to delete buffer. No buffer to delete on line 28 in file \/var\/www\/src\/Application\/ResponseEmitter\/ResponseEmitter.php."
    }
}

Which I thought was strange. This PR fixes it though.