Fixed broken filter in contest categories grid (#1628)

Closes
SoftUni-Internal/exam-systems-issues#1702

**Summary of the changes made**:
1. Handled case where parent is null
2.

Author: gogo4ds

Servers/UI/OJS.Servers.Ui/ClientApp/src/pages/administration-new/contest-categories/contestCategoriesGridColumns.tsx

@@ -95,7 +95,7 @@ const categoriesFilterableColumns: AdministrationGridColDef[] = [
         type: 'string',
         filterable: false,
         sortable: false,
-        renderCell: (params) => (
+        renderCell: (params) => params.value && (
             <ExternalLink
               to={getContestsByCategoryUrl({
                   categoryId: params.row.parentId,