During sync null attributes from template are considered

During execution of synchronization tasks, all user attributes are considered for update, with values coming from the syncing resource and from evaluated JEXL properties in the user template.

If an attribute is not present in the schema mapping of the syncing resource but is present in the user template with null value, the value is cleared and the attribute removed.

This is not correct: if an attribute of the user template has no value, it must not be considered.

A test for proving this issue is attached.