fix: make createClient callable without params

[tasshi-me]

Why

When used with session authentication, the parameter can be omitted.

What

• make input parameter of createClient optional

How to test

Checklist

• Read CONTRIBUTING.md
• Updated documentation if it is required.
• Added tests if it is required.
• Passed pnpm lint and pnpm test on the root directory.

[Copilot]

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

packages/rest/src/client/index.ts:15

• Ensure that the type definition of KintoneClientOptions accepts an empty object as valid input to avoid potential runtime type issues.

clientOptions: KintoneClientOptions = {}

packages/rest/src/client/tests/index.test.ts:5

• [nitpick] Consider adding tests to validate createClient behavior when valid clientOptions are provided, in addition to the no-argument scenario.

describe("createClient", () => {

[shabaraba]

LGTM