[osipovartem]

Summary

• Preserve a typed NULL for an OBJECT-typed placeholder when DataFusion substitutes query parameters.
• Keep the existing rejection of non-NULL scalar bindings into OBJECT.
• Assert the bound VALUES plan retains an Utf8 input field rather than falling back to Null.

Why

Rustice REST wire test connector_object_bindings_reject_scalars_and_accept_null reached the Iceberg sink with a physical Null VALUES field for INSERT INTO t(payload OBJECT) VALUES (:1) when :1 was bound as ANY = NULL; the sink rejected the schema. DataFusion's replace_params_with_values rebuilds VALUES after substitution. The target field has OBJECT metadata and Utf8 storage, but ScalarValue::Null loses that physical type.

The stock Snowflake Python Connector qmark-bound None inserted successfully into a temporary OBJECT column (1 row, 0 non-NULL payloads) in a focused live check. The oracle warehouse was suspended afterward.

Verification

• cargo +1.97.0 fmt --all -- --check
• cargo +1.97.0 test --profile ci -p datafusion-sql --test sql_integration: 642/642
• cargo +1.97.0 clippy --profile ci -p datafusion-expr -p datafusion-sql --all-targets -- -D warnings
• Rustice REST wire test connector_object_bindings_reject_scalars_and_accept_null on rev 8572b31fce6007348fcd5037cc515a0e7f3e472a: 1/1 passed through the local REST endpoint and Iceberg sink (loopback bind required an unsandboxed test run).
• Independent read-only review approved the diff with no blockers; it specifically requested this physical REST verification, now complete.

This is a narrow follow-up to #155. It does not change the common non-NULL parameter path.