fix(expr): ignore untyped NULL metadata in VALUES

[osipovartem]

Summary

• exclude untyped NULL expressions from VALUES field-metadata reconciliation
• preserve metadata inferred from concrete values regardless of whether NULL appears first or last
• retain nullability and existing rejection of conflicting concrete metadata

Why

Untyped NULL has no concrete Arrow type or field metadata. Comparing its empty metadata with metadata-bearing expressions incorrectly rejects otherwise valid VALUES columns. This surfaced in Rustice when ARRAY wire-type metadata was preserved for non-NULL array expressions.

The change is planning-only and removes metadata work for NULL expressions; it adds no per-row execution cost.

Validation

• cargo +1.95.0 test -p datafusion-expr (252 unit tests; 55 doctests passed, 3 ignored)
• cargo +1.95.0 clippy -p datafusion-expr --all-targets -- -D warnings
• cargo +1.95.0 fmt --all -- --check