fix: allow creating empty sessions

[lprnmns]

Problem

FastifyInstance.createSecureSession(data?) is declared with optional data in types/index.d.ts and is exercised without an argument in the TypeScript tests. At runtime, calling createSecureSession() throws while Session initializes its timestamp, so integrations that need an empty session cannot use the public helper.

Fix

Default omitted data to an empty object at the createSecureSession decorator boundary. Existing object arguments are unchanged, and the regression assertion confirms the returned empty session is usable.

Tests

• node --test test/decorators.test.js - passed; 3 tests
• npm test - passed; 49 unit tests with 100 percent coverage and 16 TypeScript assertions
• npm run lint - passed
• git diff HEAD^ HEAD --check - passed

Compatibility

This changes only the previously failing omitted or undefined-data case to match the existing optional TypeScript contract. Sessions created with an object retain their current behavior.

Related issue

Independent reproduction; no issue linked.