Fix edge case: rapid re-init and destroy

**Scenario**
`powerTip` is instantiated on an element that already has a `powerTip`, there is only one tooltip on the page, and `destroy` is called on that tooltip before it has a chance to show fully.

**Problem**
`session.tooltips` is `null` when `$.powerTip.destroy` tries to call `remove` on it.

**Solution**
Don't call `remove` when `session.tooltips` is `null`. Don't need to do anything there, since clearly there's nothing left to clean up.

There might be more stuff to fix if `powerTip` is instantiated multiple times on the same element, but this is the only issue I've seen with it.