ENYO-4077: Resetting unrendered DataList attempts to access nonexistent boundsCache

When passing an existing collection to a new `DataList` instance, a race condition can occur where the `DataList` tries to refresh itself before it's been rendered.

`enyo.DataList.delegates.vertical.width` and `enyo.DataList.delegates.vertical.height` try to find the (non-existent) list items that correspond with the deleted records by their index. During this search `VerticalDelegate` tries to calculate the amount of items contained within a single page, which triggers the call to `enyo.DataList.delegates.vertical.width` or `enyo.DataList.delegates.vertical.height`, which tries to retrieve the width from the `boundsCache`.

However, because the `DataList` has not been rendered yet, there are no bounds yet. The `enyo.DataList.delegates.vertical.width` and `enyo.DataList.delegates.vertical.height` methods do not anticipate this and throw an exception.

This pull request fixes the problem by returning `undefined`, which seems semantically correct and doesn't appear to cause further problems with the `DataList` either.