refactor: Fix transform casts from non-unit to unit types

[ahomescu]

Fixes the example from #1560 by emitting the following pattern:

    {
        let _ = crate::app::src::rng::randombytes(
            &raw mut seed as *mut ::core::ffi::c_uchar,
            (3 as ::core::ffi::c_int * crate::params_sphincs_haraka_128s_h::SPX_N)
                as ::core::ffi::c_ulonglong,
        );
        ()
    };

[kkysen]

We don't have things set up to add a test for this yet, right?