Custom labels fail when an XML element does not exist

[Otterfan]

Current behavior

Custom labels fail if the elements required to build that label don't exist. For example, a label like

<title>
<author>

Will fail if the item does not have an <author>, which is not uncommon.

Expected behavior

Either of the following:

• Missing elements should not trigger an error at all.
• Missing elements should trigger an error that can be suppressed if the field is prefixed with the asterisk (*) operator. The asterisk currently suppresses elements only if the element is blank but not if it is not present.

[mgobat]

Repaired for next release: if the field is prefixed with the asterisk (*) operator and the field is missing, the error is suppressed. Existing asterisk (*) operator functionality maintained.