The current Seastar RPC infrastructure lacks support
for null values in tuples in handler responses.
In this commit we add the make_default_rpc_tuple function,
which solves the problem by returning pointers to
default-constructed values for smart pointer types
rather than nulls.
The problem was introduced in this commit
2d791a5ed4. The
function `encode_replica_exception_for_rpc` used
`default_tuple_maker` callback to create tuples
containing exceptions. Callers returned pointers
to default-constructed values in this callback,
e.g. `foreign_ptr(make_lw_shared<reconcilable_result>())`.
The commit changed this to just `SourceTuple{}`,
which means nullptr for pointer types.
Fixes: #14282Closes#14352
1e851262f2