Resolve zoom on terrain with exaggeration 0

[astojilj]

The regression is originally noticed in mobile SDK mapbox/mapbox-maps-ios#1279, where the same approach is ported from #11578.
It was incorrect to (mis)use exaggeration 0 with semantics of invalid exaggeration value. Using undefined instead.

Changelog:

Resolve zoom on terrain when terrain exaggeration gets set to 0.

Issue was also reproducible using fog.html debug page

Untitled.mov

Launch Checklist

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[karimnaaji]

👍