public FieldUpdate getFieldUpdate(String fieldName) {
        Field field = documentType.getField(fieldName);
        return field != null ? getFieldUpdate(field) : null;
    }
Returns the update for a field name

@param fieldName the field name to return the update of
@return the update for the field, or null if that field has no update in this
document/src/main/java/com/yahoo/document/DocumentUpdate.java