Fix terrain "source" error being fired when using map.getStyle() with globe view

[avpeery]

Closes #11553.

In order to enable draping with globe view, we mock a terrain source like this: const mockTerrainOptions = {source: '', exaggeration: 0};. This mocked terrain is returned in the map.getStyle().

If a user decides to use set the map's style with map.getStyle() it will trigger a terrain error while validating the terrain source. It is not useful/meaningful to the user to see the mocked terrain source in the returned value from map.getStyle() and therefore should hide it from being returned in map.getStyle(). To do so, I use in the style.getTerrain() to return the value of terrain in the serialize function, which should mimic the behavior of other projections when terrain is not set (e.g. return 'undefined').

Launch Checklist

• 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>Fix terrain error being fired when using map.getStyle() with globe view</changelog>

[ansis]

The overall direction of this is good, but we shouldn't change the behavior of the public map.getTerrain(). It's currently documented as

Returns the terrain specification or null if terrain isn't set on the map.`

We should maintain that behavior