fix: global search null exception

Author: mfan127

src/Apps/StatCan.OrchardCore.Radar/Controllers/ListController.cs

@@ -105,7 +105,7 @@ private async Task<IEnumerable<ContentItem>> GetContentItems(string type, string
             // Prepare the parameters
             IDictionary<string, object> parameters = new Dictionary<string, object>();
             parameters.Add("Type", type);
-            parameters.Add("Term", searchText.ToLower());
+            parameters.Add("Term", searchText != null ? searchText.ToLower() : "");
 
             var results = await _queryManager.ExecuteQueryAsync(query, parameters);