Fix invalid LngLat with no currently loaded DEM tiles

[SnailBones]

Launch Checklist

Closes #10610

When terrain is enabled and the map is quickly zoomed, map.elevation.visibleDemTiles occasionally returns an empty array while awaiting DEM tile loading. This previously caused getBounds to return null values, As explained here by @ted-piotrowski:

If elevation.visibleDemTiles is empty array and elevation.exaggeration() > 1 then minmax.min * elevation.exaggeration() will equal Infinity, possibly producing a NaN down the line:

This PR adds a fix by defaulting to the 2D getBounds behavior if no terrain is available.

This leaves undefined behavior in the application, where getBounds returns different results depending on whether terrain is loaded or not. It's also likely that if 1+ but not all DEM tiles are loaded, the bounds returned could be insufficient to completely cover all terrain, though this did not nor will they now throw an error.

I suspect that these cases are subtle enough to be only a minor issue. If we decide to address this in the future, we should consider the performance cost of delaying getBoundscalls until terrain is loaded.

• briefly describe the changes in this PR
• write tests for all new functionality
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Fixed 'getBounds' sometimes returning invalid LngLat when zooming on a map with terrain.</changelog>

[karimnaaji]

One nit but looks good to me otherwise.

[karimnaaji]

Good catch 👍