[Term Entry] C++ Unordered-sets: empty()

[ylevental]

Description

New term entry for the empty() method under C++ unordered-set.

File: content/cpp/concepts/unordered-set/terms/empty/empty.md

Entry includes:

• Description of the .empty() method
• Syntax section with parameters and return value
• Example showing basic usage with output
• Codebyte example with compilable loop-based processing

Issue Solved

Closes #8048

Type of Change

• Adding a new entry
• ~~Editing an existing entry (fixing a typo, bug, issues, etc)~~
• ~~Updating the documentation~~

Checklist

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