fix(report_utils): ensure that delimiter and separator can't be empty

This results in them setting as `undefined`, which ends up as a string
in python, and the delimiter ends up as `'u'`, and the separator as
`'undefined'`.

(cherry picked from commit af611d4)
Signed-off-by: Akhil Narang <me@akhilnarang.dev>

frappe/public/js/frappe/views/reports/report_utils.js

@@ -256,8 +256,12 @@ frappe.report_utils = {
 
 		dialog.fields_dict["file_format"].df.onchange = () => update_csv_preview(dialog);
 		dialog.fields_dict["csv_quoting"].df.onchange = () => update_csv_preview(dialog);
-		dialog.fields_dict["csv_delimiter"].df.onchange = () => update_csv_preview(dialog);
-
+		dialog.fields_dict["csv_delimiter"].df.onchange = () => {
+			if (!dialog.get_value("csv_delimiter")) {
+				dialog.set_value("csv_delimiter", ",");
+			}
+			update_csv_preview(dialog);
+		};
 		return dialog;
 	},