Avoid clippy::unit_arg in generated builder code for unit-typed properties

[danieleades]

Problem

For struct properties whose type is the unit type () (produced by null schemas), builder-mode codegen emits expressions that pass a unit value to Ok(..):

• in impl Default for builder::T: field: Ok(Default::default())
• in impl From<super::T> for builder::T: field: Ok(value.field)

Both trip clippy's warn-by-default unit_arg lint. Generating builder-mode output for the typify-impl/tests/github.json fixture produces 105 such warnings. cargo-typify's generated #![allow(..)] prelude doesn't cover unit_arg, and consumers using the macro or build.rs get the warnings in their own builds.

Fix

Detect unit-typed properties in output_struct and emit Ok(()) directly in the builder's Default and From impls. This is semantically identical: reading or defaulting a unit field always yields () and has no effects.

Verified by compiling the github.json builder output (with the #![allow(..)] prelude stripped) under cargo clippy: unit_arg warnings drop from 105 to 0. No existing golden files change, since none of them exercise a unit-typed property in builder mode.

Trade-offs

None behavioral; only the spelling of the generated initializers for unit-typed fields changes.