Skip undefine-attribute for attributes that doesn't exist as a result of alternatives checking

There are some cases where the attributes listed in the alternatives metadata doesn't exist (see WFCORE-4042).
In HAL when an user changes a form and save, there is a check and operation to undefine the attribute listed in the "alternatives" metadata, if the attribute doesn't exist then there is an exception. We should skip the undefine operation if the attribute doesn't exist.