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/pallet/mod.rs

@@ -339,7 +339,7 @@ pub mod pallet {
 	/// TWOX-NOTE: SAFE since `AccountId` is a secure hash.
 	#[pallet::storage]
 	pub type Payee<T: Config> =
-		StorageMap<_, Twox64Concat, T::AccountId, RewardDestination<T::AccountId>, ValueQuery>;
+		StorageMap<_, Twox64Concat, T::AccountId, RewardDestination<T::AccountId>, OptionQuery>;
 
 	/// The map from (wannabe) validator stash key to the preferences of that validator.
 	///
@@ -1911,7 +1911,7 @@ pub mod pallet {
 			ensure!(
 				(Payee::<T>::get(&ledger.stash) == {
 					#[allow(deprecated)]
-					RewardDestination::Controller
+					Some(RewardDestination::Controller)
 				}),
 				Error::<T>::NotController
 			);
@@ -1948,7 +1948,7 @@ pub mod pallet {
 						// `Controller` variant, skip deprecating this account.
 						let payee_deprecated = Payee::<T>::get(&ledger.stash) == {
 							#[allow(deprecated)]
-							RewardDestination::Controller
+							Some(RewardDestination::Controller)
 						};
 
 						if ledger.stash != *controller && !payee_deprecated {