Tooltip fixes (getLabelAndValue on null controller, null getParsed) (#11596)

Tooltip fixes (getLabelAndValue on null controller, null getParsed) (#11596)

* Fix for getLabelAndValue on null controller

I encountered #11315 under the following circumstances:

1. Position the cursor over the chart area, such that it causes a
   tooltip to be shown.
2. Move the cursor out of the chart area, such that the tooltip remains
   visible.
3. Cause the chart contents to be changed, such that the dataset
   referenced by the active tooltip element is no longer valid.
4. Move the mouse again.  This triggers an `inChartArea = false` event,
   so it reuses the previous, now invalid, active elements.

This fixes #11315 under the circumstances for which I've reproduced it,
but there may be others.

* Further fixes for elements added / changed

This possibly fixes #11365.