fix(journal): ensure cost_center_id exists

This commit adds in a NULL check to the posting journal's cost center id
checks.

Closes #6214.

client/src/js/services/TransactionService.js

@@ -125,7 +125,7 @@ function TransactionService($http, util, Modal, Accounts) {
         return ERROR_INVALID_DEBITS_AND_CREDITS;
       }
 
-      const hasCostCenters = angular.isDefined(row.cost_center_id);
+      const hasCostCenters = angular.isDefined(row.cost_center_id) && row.cost_center_id !== null;
       if (hasCostCenters && !Accounts.isIncomeOrExpenseAccountTypeId(row.account_type_id)) {
         return ERROR_COST_CENTER_ON_OTHER_ACCOUNT;
       }