Allow NULL in Episode runtime #293

Some eposodes to not have a runtime.

When calling new TvRepository($tmdb_client)->load(30984) I get:

TypeError: Tmdb\Model\Tv\Episode::setRuntime(): Argument #1 ($runtime) must be of type int, null given, called in /app/vendor/php-tmdb/api/lib/Tmdb/Common/ObjectHydrator.php on line 52 in Tmdb\Model\Tv\Episode->setRuntime() (line 525 of /app/vendor/php-tmdb/api/lib/Tmdb/Model/Tv/Episode.php).

Proposed solution:

Allow NULL in \Tmdb\Model\Tv\Episode::setRuntime