Draw a pivot with a PIVOT LAST property in the web client #1968

Description

In the web client a pivot table with a PIVOT LAST property is not drawn: opening it shows TypeError: Cannot read properties of null and an empty table, and once the pivot's grouping comes from the server the table stays empty with another TypeError.

FORM items
    OBJECTS i = Item PIVOT
    PROPERTIES(i) READONLY name, quantity PIVOT LAST (name(i))
;

With three items whose quantities are 100, 200 and 300, the total of quantity should be the quantity of the item last by name, but nothing is shown.

Reason

PIVOT LAST could not be used in the web client at all: a form with such a property in the pivot view fails as soon as it is opened.

Fix

A pivot with a PIVOT LAST property is drawn, and the property's aggregated value is taken from the row that is last in the order of the listed expressions, as documented.