fix(db-postgres): incorrect results querying json field using exists

[DanRibbens]

Description

There is a surprising lack of tests on querying json, I will continue to add them in another PR. #3839

• [x] I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] I have added tests that prove my fix is effective or that my feature works
• [x] Existing test suite passes locally with my changes
• [ ] I have made corresponding changes to the documentation