Sync crypto-square with problem specifications

[ManasDasri]

Adds the missing 'normalization results in empty plaintext' canonical case and updates the reimplemented 54-character case (description corrected from 7 to 8 chunks, same input/expected values).

This also fixes a real bug in Example.vb: Ciphertext checked plaintext.Length (the raw input) before the empty-check, so inputs that normalize to an empty string (e.g. '... --- ...', all punctuation) skipped the early return and crashed trying to build rows/columns from nothing. Now it checks the normalized string's length instead.

Verified via pwsh ./bin/test.ps1 crypto-square - all tests pass.

Closes #434

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[BNAndras]

Looks good. The CI is failing due to an unrelated exercise having a flaky test. I'll look into that separately, but I can confirm from the CI log that Crypto Square passes all its tests.

Since you updated the example solution to pass the new tests, feel free to add your GH username to the contributors array in the exercise's .meta/config.json file. That'll get you some more rep and show you on the website as a contributor to this exercise.

[ManasDasri]

Thanks for taking a look and for confirming that Crypto Square is passing all the tests! 🙌

I'll add my GitHub username to the contributors array as suggested. And thanks for looking into the unrelated flaky test as well!