fix: Cannot use an Option with a UDT as T

[willemneal]

With this change you get the following compiler error:

error[E0277]: the trait bound `soroban_env_host::xdr::ScVal: TryFrom<&std::option::Option<InnerUdt>>` is not satisfied
  --> soroban-sdk/src/tests/contract_udt_option.rs:11:1
   |
11 | #[contracttype]
   | ^^^^^^^^^^^^^^^ the trait `From<InnerUdt>` is not implemented for `soroban_env_host::xdr::ScVal`, which is required by `&std::option::Option<InnerUdt>: TryInto<_>`
   |
   = help: the following other types implement trait `From<T>`:
             `&'a soroban_env_host::xdr::ScVal` implements `From<ScValObjRef<'a>>`
             `soroban_env_host::xdr::ScVal` implements `From<&()>`
             `soroban_env_host::xdr::ScVal` implements `From<&address::Address>`
             `soroban_env_host::xdr::ScVal` implements `From<&bool>`
             `soroban_env_host::xdr::ScVal` implements `From<&bytes::Bytes>`
             `soroban_env_host::xdr::ScVal` implements `From<&i128>`
             `soroban_env_host::xdr::ScVal` implements `From<&i32>`
             `soroban_env_host::xdr::ScVal` implements `From<&i64>`
           and 36 others
   = note: required for `InnerUdt` to implement `Into<soroban_env_host::xdr::ScVal>`
   = note: required for `soroban_env_host::xdr::ScVal` to implement `From<&std::option::Option<InnerUdt>>`
   = note: 1 redundant requirement hidden
   = note: required for `&std::option::Option<InnerUdt>` to implement `Into<soroban_env_host::xdr::ScVal>`
   = note: required for `soroban_env_host::xdr::ScVal` to implement `TryFrom<&std::option::Option<InnerUdt>>`
   = note: required for `&std::option::Option<InnerUdt>` to implement `TryInto<soroban_env_host::xdr::ScVal>`
   = note: this error originates in the attribute macro `contracttype` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0277`.