Fixing error that occurs when an item in an Observe.List contains a comp...

...ute and is removed

If an Observe.List item contains a compute that does not have any bindings, removing that item from the list results in a JS error in [compute.js](https://github.com/bitovi/canjs/blob/master/observe/compute/compute.js#L360)

This occurs when the compute does not have any bindings, so `on` is never called, leaving `computedData === undefined`.

Fix is to check that computedData is truthy before calling teardown()