[Concept Entry] Javascript: Nullish Coalescing

[hitomipupil]

Description

• Added Nullish Coalescing concept for Javascript

Issue Solved

Closes #5804

Type of Change

• Adding a new entry

Checklist

• All writings are my own.
• My entry follows the Codecademy Docs style guide.
• My changes generate no new warnings.
• I have performed a self-review of my own writing and code.
• I have checked my entry and corrected any misspellings.
• I have made corresponding changes to the documentation if needed.
• I have confirmed my changes are not being pushed from my forked main branch.
• I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• I have linked any issues that are relevant to this PR in the Issues Solved section.

[CLAassistant]

All committers have signed the CLA.

[mamtawardhani]

Thank you for contributing to Codecademy Docs @hitomipupil 😄

The entry looks good for a second round of review!🚀

[Sriparno08]

Looks good, @hitomipupil!

[github-actions]

👋 @hitomipupil
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