False sniff for missing comment of a @throws directive

According to Drupal's documentation standards https://www.drupal.org/node/1354#throws , the comment for a throws directive is optional and only should be added if adds clarity to the use of an exception. Currently Drupal.Commenting.FunctionComment.EmptyThrows highlights the missing comment line as an error. This issue is to correct the false sniff for this condition.

This sniff condition was discovered while performing a code review for #2624640: Fixes to docs and code style for migrate/src/*.php files.