Validate removed/empty timestamp values as true

edit/js/dariah.workflow.table.js

@@ -51,7 +51,7 @@ function handleHeaderCells(row) {
 function getDateFieldProps(){
 	return {
 		validator: function(value, callback){ //custom date validator
-			if (xsdGYear.test(value) || xsdGYearMonth.test(value) || xsdDateOrDateTime.test(value)) {
+			if (!value || xsdGYear.test(value) || xsdGYearMonth.test(value) || xsdDateOrDateTime.test(value)) {
 				callback(true);
 			} else callback(false);
 		},