Fix: Guard domestic hot water mapping against undefined lists

[sentry]

This PR addresses a TypeError (Cannot read properties of undefined (reading '0')) occurring in mapHotWaterSourcesData when the waterStorage section of the domestic hot water state is incomplete.

The root cause was that resolveState omits sections that are not marked as complete, leading to state.domesticHotWater.waterStorage being undefined. The code then attempted to access waterStorage[0] without optional chaining, causing the crash.

Changes made in mapping/domesticHotWaterMapper.ts:

• Changed state.domesticHotWater.waterStorage[0] to state.domesticHotWater.waterStorage?.[0] to safely handle undefined values.
• Added ?? [] guards to state.domesticHotWater.pipework and state.domesticHotWater.hotWaterOutlets when calling .filter() to prevent similar TypeErrors if these lists are undefined.

Fixes EPB-CHECK-PART-L-FRONTEND-1B

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