Allow default (no value) sort function in hippofacnav:sortfunction

When configuring result set sorting for faceted navigation, a sort function can optionally be specified for each sort property.

For example:

• hippofacnav:sortby = {'myhippoproject:title', 'myhippoproject:subtitle'}
• hippofacnav:sortfunction = {'lower-case', 'normalize'}

If no hippofacnav:sortfunction property exists, default sort function is used. But if hippofacnav:sortfunction exists, the only allowed values are "lower-case" and "normalize". In other words, it is not possible to have default sort function for one sortyby property, and e.g. "lower-case" for another.

For example, when you have:

• hippofacnav:sortby = {'myhippoproject:date', 'myhippoproject:title'}

It would make sense to use "lower-case" or "normalize" for "title". But for "date", neither makes sense.

If an empty string would be accepted like this:

• hippofacnav:sortfunction = {'', 'normalize'}

The default sort function would be used for date.