TextTransformer: Allow yearly and weekly rules without a start date

[NlL5]

The following code would not work. It would throw Fatal error: Uncaught Error: Call to a member function format() on null in /srv/www/Projekte/Web/mms-web/vendor/simshaun/recurr/src/Recurr/Transformer/TextTransformer.php:110 :

$textTransformer = new TextTransformer();
echo $textTransformer->transform(new Rule('FREQ=YEARLY'));

It will only work with and additional BYDAY, BYMONTHDAY,... or a start date in the Rule. However, I think it makes sense to be able to retrieve the text "yearly" for the above Rule. (As returned for the "daily", "monthly" or "hourly" rule without start date.)

The same applies to "weekly".

I added a simple check to fix the PHP error.