monero-serai: make it clear that not providing a change address is fingerprintable (#472)

* Make it clear not providing a change address is fingerprintable

When no change address is provided, all change is shunted to the
fee. This PR makes it clear to the caller that it is fingerprintable
when the caller does this.

* Review comments

tests/full-stack/src/tests/mint_and_burn.rs

@@ -395,7 +395,7 @@ async fn mint_and_burn_test() {
             ),
             1_100_000_000_000,
           )],
-          Some(Change::new(&view_pair, false)),
+          Change::new(&view_pair, false),
           vec![Shorthand::transfer(None, serai_addr).encode()],
           rpc.get_fee(Protocol::v16, FeePriority::Low).await.unwrap(),
         )