fix: guard verticalExaggeration against undefined to prevent Cesium crash

[ZTongci]

Bug

Enabling terrain for the first time when verticalExaggeration has not been
configured causes a Cesium crash:

  DeveloperError: scale must be a finite number.
      at VerticalExaggeration.getHeight
      at ScreenSpaceCameraController.update
      at Scene.initializeFrame

Root Cause

The previous code always passed property?.scene?.verticalExaggeration directly to <Scene>. When this value was never configured, it was always undefined — Resium saw no change between renders and never wrote anything to the Cesium scene, so Cesium kept its internal default of 1.0 untouched.

The terrain-disable fix changed this: when terrain is disabled we now explicitly pass 1, and when terrain is re-enabled we pass property?.scene?.verticalExaggeration. If no exaggeration is configured, the prop transitions from 1 (number) → undefined. Resium detects this as a prop change and actively calls scene.verticalExaggeration = undefined. Cesium's VerticalExaggeration.getHeight requires scale to be a finite number : undefined is not, so it throws a DeveloperError and halts rendering.

Fix

Add ?? 1 so undefined is never passed when terrain is enabled but no
exaggeration value has been set:

// Before
property?.terrain?.enabled ? property?.scene?.verticalExaggeration : 1

// After
property?.terrain?.enabled ? (property?.scene?.verticalExaggeration ?? 1) : 1

This ensures the prop is always a valid finite number, preventing Resium
from writing undefined into the Cesium scene.

[bnimit]

LGTM!