`test_display_pg_json` currently fails with default features

[AdamGS]

Describe the bug

The default feature set for datafusion-expr makes the test to fail, getting it to pass relies on feature unification for serde_json.

To Reproduce

cargo test -p datafusion-expr test_display_pg_json

Expected behavior

Ideally, tests should pass with default features, or at least make it clear they require enabled features.

Additional context

No response

[AdamGS]

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