Allow NULL in TV season's air date

[mikeshiyan]

Problem

Calling new TvRepository($tmdb_client)->load(1466) I get:

Deprecated function: DateTime::__construct(): Passing null to parameter #1 ($datetime) of type string is deprecated in Tmdb\Model\Tv\Season->setAirDate() (line 134 of /app/vendor/php-tmdb/api/lib/Tmdb/Model/Tv/Season.php).

Because the show's special season has no air date - https://www.themoviedb.org/tv/1466-joey/seasons

Proposed solution

Allow NULL in season's air date, like it's done in episode.