[T2580] Fix handling null values in mysql

[Lagovas]

On decryption our envelope_detector returned []byte{} on nil values instead leaving nil. MySQL encoded differently empty strings and NULL values. NULL values should have 0xfb value. So on Acra side when it receive null value as 0xfb then it changed to empty array and encode it as LengthEncoded String or Integer that are incorrect.

So in this PR removed processing null data in crypto envelope. If it will be empty array, it will be returned as empty array too.

Checklist

• [+] Change is covered by automated tests
• [+] The coding guidelines are followed
• [+] Public API has proper documentation in the Acra documentation site or has PR on documentation repository with new changes
• [+] CHANGELOG.md is updated (in case of notable or breaking changes)
• [+] CHANGELOG_DEV.md is updated
• [+] Benchmark results are attached (if applicable)
• + are up-to-date (in case of API changes)