private void deserializeFieldValue(FieldBase field, FieldValue value) {
        value.deserialize(field instanceof Field ? (Field)field : null, this);
    }
Optional test and set condition. Common for document/update/remove elements
This variable is either set in VespaXMLFieldReader#read (reader for document)
or in VespaXMLUpdateReader#read (reader for update).
document/src/main/java/com/yahoo/vespaxmlparser/VespaXMLFieldReader.java