refactor: eliminate remaining non-null assertions in ChargingStation and OCPP16Servic...

refactor: eliminate remaining non-null assertions in ChargingStation and OCPP16ServiceUtils

- ChargingStation: replace 8 non-null assertions with optional chaining,
  nullish coalescing defaults, and null guards
- OCPP16ServiceUtils: add type narrowing guards for composite charging
  schedules, extract connector status variables with null checks,
  replace all 14 non-null assertions with safe access patterns

Zero non-null assertions remain across the entire src/ directory.