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ARROW-9778: [Rust] [DataFusion] Implement Expr.nullable() and make consistent between logical and physical plans

- Implements `Expr.nullable()`
- Updates some PhysicalExpr nullable methods to be consistent
- Updates `sql.rs` integration test to assert that schema is same between logical and physical plans

Closes #8005 from andygrove/nullability

Lead-authored-by: Andy Grove <andygrove73@gmail.com>
Co-authored-by: Andy Grove <andygrove@users.noreply.github.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>