refactor(build): fix a small issue with TS missing type

Author: Ghislain Beaulac

src/app/modules/angular-slickgrid/services/export.service.ts

@@ -211,7 +211,7 @@ export class ExportService {
       const isEvaluatingFormatter = (columnDef.exportWithFormatter !== undefined) ? columnDef.exportWithFormatter : this._exportOptions.exportWithFormatter;
 
       // did the user provide a Custom Formatter for the export
-      const exportCustomFormatter: Formatter = (columnDef.exportCustomFormatter !== undefined) ? columnDef.exportCustomFormatter : undefined;
+      const exportCustomFormatter: Formatter | undefined = (columnDef.exportCustomFormatter !== undefined) ? columnDef.exportCustomFormatter : undefined;
 
       let itemData = '';