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/nomination-pools/test-staking/src/lib.rs

@@ -214,7 +214,10 @@ fn pool_slash_e2e() {
 			]
 		);
 
-		assert_eq!(Payee::<Runtime>::get(POOL1_BONDED), RewardDestination::Account(POOL1_REWARD));
+		assert_eq!(
+			Payee::<Runtime>::get(POOL1_BONDED),
+			Some(RewardDestination::Account(POOL1_REWARD))
+		);
 
 		// have two members join
 		assert_ok!(Pools::join(RuntimeOrigin::signed(20), 20, 1));