TLS - Fix empty string pop_back() if all elliptic curve point formats are grease values

[Zadamsa]

Summary

Fixed undefined behavior in TLS extension string concatenation when all values are GREASE

Details

Check if string is empty before pop_back(). Similar code sections were checked for the same class of errors

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✅ Checks

• [ ] Relevant documentation was updated (if needed)
• CI pipeline passes (build, tests, lint, static analysis)
• New code follows clang-tidy rules
• The change is clearly related to a specific issue / requirement
• [ ] Documentation (e.g. Doxygen) is updated if needed
• The PR is rebased on the latest master
• [] No unrelated changes are included

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