Fix removeSource throwing TypeError in Terrain.update under globe projection

[mhayk]

Fixes #13665
Fixes #13703

Summary

Calling map.removeSource(id) on a globe-projection map whose style has no terrain block throws TypeError: Cannot read properties of undefined (reading 'get') synchronously, originating in Terrain.update.

Root cause

Style#_createTerrain left terrain.properties uninitialised — it was only assigned by Terrain#recalculate, which first runs when Style#update is called from Map#_render. Under globe projection, setTerrainForDraping() creates a terrain on style load, so Map#removeSource → Map#_updateTerrain → Terrain#update could dereference terrainProps.get('source') before the first render frame evaluated the properties.

Fix

Evaluate the terrain properties synchronously in Style#_createTerrain, so they are always available to consumers regardless of whether a render frame has run yet.

Testing

Added a unit test in test/unit/terrain/terrain.test.ts that reproduces the report (globe projection, style without terrain, addSource + removeSource before the first render). It fails with the exact TypeError from the issue without the fix and passes with it.

Launch Checklist

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[ibesora]

Thanks for this @mhayk , I'll take a look and report back