Fix: Prevent TypeError when mechanical ventilation data is incomplete

[sentry]

This PR addresses a TypeError: Cannot read properties of undefined (reading 'map') that occurred in mapping/infiltrationVentilationMapper.ts.

The root cause was that mapMechanicalVentilationData attempted to call .map() on state.infiltrationAndVentilation.mechanicalVentilation without a null guard. The resolveState function (in stores/resolve.ts) only populates this property if the associated form (/infiltration-and-ventilation/mechanical-ventilation) is marked as complete. If a user visited this page but did not complete it before attempting to calculate, mechanicalVentilation would be undefined, leading to the crash.

The fix involves adding a null-coalescing operator (?? []) to ensure that .map() is always called on an array, even if the property is undefined. This pattern is already used elsewhere in the codebase (e.g., for ductwork). The same fix has also been applied to state.infiltrationAndVentilation.vents to prevent a similar potential crash.

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

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