Keep annotated untyped NULL neutral during CASE coercion

[osipovartem]

Summary

• Keep untyped NULL neutral when type-only CASE coercion would otherwise retain partial metadata and erase matching extension metadata.
• Cover matching and conflicting NULL annotations before and after coercion.
• Backport the fix from fix: preserve field metadata for simple CASE branches apache/datafusion#26145 with Arrow 59 / DF55 API compatibility.

Validation

• cargo +1.97.0 test -p datafusion-expr -p datafusion-optimizer --lib --locked -q (255 + 764 passed)
• cargo +1.97.0 clippy -p datafusion-expr -p datafusion-optimizer --lib --tests --locked -q
• cargo +1.97.0 fmt --all -- --check; git diff --check
• Independent read-only review: approved, no blocking findings.