fix(ffi): Add implementation of Niche for Option<Box<R>> (#5094)

Signed-off-by: Marin Veršić <[email protected]>

crates/iroha_ffi/src/option.rs

@@ -1,5 +1,6 @@
 //! Logic related to the conversion of [`Option<T>`] to and from FFI-compatible representation
 
+use alloc::boxed::Box;
 use core::mem::MaybeUninit;
 
 use crate::{
@@ -53,6 +54,13 @@ where
     const NICHE_VALUE: Self::ReprC = core::ptr::null_mut();
 }
 
+impl<R, C> Niche<'_> for Box<R>
+where
+    Self: FfiType<ReprC = *mut C>,
+{
+    const NICHE_VALUE: Self::ReprC = core::ptr::null_mut();
+}
+
 impl<R: OptionIr> Ir for Option<R> {
     type Type = R::Type;
 }