externalPointerEvents doesn't trigger the legend extra value update

[markov00]

Describe the bug
If externalPointerEvents tooltip visiblility is set to false, the legend extra value is no longer updated

To Reproduce
Steps to reproduce the behavior:

1. Go to https://codesandbox.io/s/loving-poincare-0mpuz?file=/src/App.tsx
2. Move the mouse on the top most chart, the bottom chart legend updates its value.
3. Move the mouse on the bottom most chart, the legend extra value of the top chart doesn't change

Expected behaviour
Independentely from the tooltip visibility, an external pointer events should update the legend extra value with the actual value computed from the external pointer event x position.

Version (please complete the following information):

• Elastic Charts: current master

Additional context
Verified that on version 7.6 and on master in Kibana and seems to be a regression (at least in TSVB that is the only app that use the sync with the hidden tooltip)

Errors in browser console
n/a

[nickofthyme]

Duplicate of #992