#610 Add null check for stellarTransactionId in vaults.service.ts

[vic-Gray]

Area: bug · Effort: XS\n\nWhat: Guard against null/undefined stellarTransactionId before using it.\n\nWhy: A null dereference here would cause a 500 error on an otherwise valid request.\n\nHints:\n- backend/src/vaults/vaults.service.ts line 196