SDK: make `StorageMapAccess::get` return `Option<V>`

Defined at https://github.com/0xMiden/compiler/blob/9f7ec43d21f81c17946258a8b4dc3168690f3548/sdk/base/src/types/storage.rs?plain=1#L33.

### Why
The underlying `storage::get_map_item` returns zero word ([0, 0, 0, 0]) in case there is no value associated with the given key. Let's make `get` more in sync with idiomatic Rust.