Additional empty attribute check in application (#3198)

* attribute dst_attr was checked for null value
* it needs to be checked for empty value too as it is passed from GUI

perun-registrar-lib/src/main/java/cz/metacentrum/perun/registrar/impl/RegistrarManagerImpl.java

@@ -3658,7 +3658,7 @@ private Candidate createCandidateFromApplicationData(Application app) {
 		// put application data into Candidate
 		final Map<String, String> attributes = new HashMap<>();
 		jdbc.query("select dst_attr,value from application_data d, application_form_items i where d.item_id=i.id "
-				+ "and i.dst_attr is not null and d.value is not null and app_id=?",
+				+ "and i.dst_attr is not null and i.dst_attr <> '' and d.value is not null and app_id=?",
 			(resultSet, i) -> {
 				attributes.put(resultSet.getString("dst_attr"), resultSet.getString("value"));
 				return null;