[Refactor] Simplify extractMyshopifyHandle using optional chaining

[gonzaloriestra]

What has changed?

Refactored extractMyshopifyHandle in packages/cli-kit/src/public/common/url.ts to simplify control flow using optional chaining instead of an explicit null check and ternary operator.

How to test your changes?

CI

Checklist

• I have read the Contributing Guidelines
• I have considered the impact of my changes on performance
• I have added/updated tests for my changes
• I have targeted main branch

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PR created automatically by Jules for task 5073782902792885091 started by @gonzaloriestra

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