Ensure all $args to csvToArray() are strings

[claudiu-cristea]

I'm getting

[error]  Message: Deprecated function: explode(): Passing null to parameter #2 ($string) of type string is deprecated in Drush\Utils\StringUtils::csvToArray() (line 25 of vendor/drush/drush/src/Utils/StringUtils.php).
Drush\Utils\StringUtils::csvToArray(NULL) (Line: 175)
Drush\Drupal\Commands\core\EntityCommands->getQuery('taxonomy_term', Array, Array) (Line: 56)
Drush\Drupal\Commands\core\EntityCommands->delete('taxonomy_term', NULL, Array)

The reason is that $options['exclude'] is passed as null to StringUtils::csvToArray().

Let's make StringUtils::csvToArray() strong