Removes the Default implementation for RewardDestination (#2402)

This PR removes current default for `RewardDestination`, which may cause
confusion since a ledger should not have a default reward destination:
either it has a reward destination, or something is wrong. It also
changes the `Payee`'s reward destination in storage from `ValueQuery` to
`OptionQuery`.

In addition, it adds a `try_state` check to make sure each bonded ledger
have a valid reward destination.

Closes https://github.com/paritytech/polkadot-sdk/issues/2063

---------

Co-authored-by: command-bot <>
Co-authored-by: Ross Bulat <ross@parity.io>

substrate/frame/staking/src/ledger.rs

@@ -126,7 +126,7 @@ impl<T: Config> StakingLedger<T> {
 	/// default reward destination.
 	pub(crate) fn reward_destination(
 		account: StakingAccount<T::AccountId>,
-	) -> RewardDestination<T::AccountId> {
+	) -> Option<RewardDestination<T::AccountId>> {
 		let stash = match account {
 			StakingAccount::Stash(stash) => Some(stash),
 			StakingAccount::Controller(controller) =>
@@ -137,7 +137,7 @@ impl<T: Config> StakingLedger<T> {
 			<Payee<T>>::get(stash)
 		} else {
 			defensive!("fetched reward destination from unbonded stash {}", stash);
-			RewardDestination::default()
+			None
 		}
 	}