Bug 42550: Remove JS error from report list if none exist

If there are no saved SQL reports, the "rtable" variable is empty and
triggers an error when called later.
Uncaught TypeError: can't access property "DataTable", rtable is
undefined

Test plan:
/cgi-bin/koha/reports/guided_reports.pl?op=list
Without reports
=> Without this patch there is a JS error in the console
=> With the patch the error is gone

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>

koha-tmpl/intranet-tmpl/prog/en/modules/reports/guided_reports_start.tt

@@ -2291,7 +2291,7 @@
                     },
                     table_settings
                 );
-                let table_dt = rtable.DataTable();
+                let table_dt = rtable?.DataTable();
 
                 $("#tabs a[data-bs-toggle='tab']").on("shown.bs.tab", function (e) {
                     tabsInit($(e.target).parent(), rtable);