d85b3f56ac
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