[FIX] calendar: compare display_name for empty recordset

When sorting by a many2many fields (e.g. partner_ids) with one of the record
having an empty value on the sorted field, the comparison method used to compare
and empty record (e.g. res.partner()) and a name_get result (e.g. "Agrolait").

This is because the sort_field was initialized with the result of self[field]
but never assigned a new value below.

Fallback on an empty string when no record is found

Fixes #26908