fix(ngValue): correctly update the `value` property when `value` is undefined

Previously, when the expression evaluated to `undefined` the `value` property
was not updated. This happened because jqLite/jQuery's `prop(_, undefined)` is
treated as a getter, thus not apdating the property.

This commit fixes it by setting the property to `null` instead.
(On IE9 we use `''` - see inline comments for more info.)

Fixes #15603

Closes #15605