TT#72750 - Allow empty value for week day number in timeset event

* Week day number was previuosly restricted to
	   a certain format because of JS failing to
	   parse other formats an this caused it to
	   fail on empty value; adding empty values is
	   now possible

Change-Id: I14d2b9627b7082821d80fe51bf6021461bd5f961
(cherry picked from commit b76e462a1b)

lib/NGCP/Panel/Form/TimeSet/EventAdvanced.pm

@@ -695,7 +695,7 @@ sub validate_byday {
     my ($self, $field) = @_;
 
     if (my $weekdaynumber = $field->field('weekdaynumber')) {
-        unless ($weekdaynumber->value =~ /^([+-]?\d)(MO|TU|WE|TH|FR|SA|SO)$/) {
+        if ($weekdaynumber->value && $weekdaynumber->value !~ /^([+-]?\d)(MO|TU|WE|TH|FR|SA|SO)$/) {
             $weekdaynumber->add_error($weekdaynumber->label . ' should be like (\+|-)?\d*(MO|TU|WE|TH|FR|SA|SO) (e.g. 5FR)');
             return 0;
         }