Fix Notice: Undefined index: operator in addConditionGroup

[MurzNN]

When executing queries with ConditionGroup, that doesn't contain operator (that is not required), Drupal shows the notice:

Notice: Undefined index: operator in Drupal\graphql_search_api\Plugin\GraphQL\Fields\SearchAPISearch->addConditionGroup() (line 172 of web/modules/contrib/graphql_search_api/src/Plugin/GraphQL/Fields/SearchAPISearch.php).

Here is fix for this notice. Instead of if checking, I use the ?? construction, that must be available in PHP 7.0+

Also value may be not required too (for example, if operator = "IS_NULL"), so I add NULL replacement to it.