Fixed issue with null values in MySQL prepared statements

[Zhaars]

Fixed issue regarding handling null values during PreparedStatements in MySQL;

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)
• Example projects and code samples are up-to-date (in case of API changes)