Mark scalar subquery results nullable

[osipovartem]

Summary

• mark scalar subquery expressions nullable because SQL returns NULL when the subquery produces zero rows
• preserve the inner field type and metadata while forcing nullable output schema
• cover both ExprSchemable::nullable and ExprSchemable::to_field with a focused unit test

Why

Rustice's live Snowflake compatibility oracle exposed a valid scalar subquery over an empty result. Execution produced NULL, but DataFusion propagated the non-nullable inner field and Arrow rejected the result with:

Column 'scalar_empty' is declared as non-nullable but contains null values

Validation

• cargo +1.97.0 test -p datafusion-expr -q (249 unit tests and 55 doc/integration tests passed; 3 ignored)
• cargo +1.97.0 clippy -p datafusion-expr --all-targets -- -D warnings
• cargo +1.97.0 fmt --all -- --check