Fix handling null values for postgresql in binary mode

[Lagovas]

Previously we missed the logic of handling NULL values which should be encoded differently than empty strings. We encoded them as empty strings instead of placing -1 value according to wire protocol This should fix #591.

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)