Export placeholder missing values as empty in CSV/JSON

[Eepsita12]

Description

This PR updates CSV and JSON export behavior to ensure placeholder strings such as «No Value», «Empty Value», and «No Type» are treated as missing data and exported as empty/null values instead of literal strings.

Validation

• Verified via existing export-related tests (transformToCsv and transformToJson)
• Manual review confirms placeholder values are converted to empty cells in CSV
• Full test suite was executed locally; failures observed were OS path and
  locale-specific (Windows / Indian numbering format), unrelated to this change

Related Issues

Fixes #1481

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0
  license.
• I have run pnpm checks to ensure code compiles and meets standards.
• I have run pnpm test to check if all tests are passing.
• I have covered new added functionality with unit tests if necessary.
• I have added an entry in the Changelog.md.