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/mock.rs

@@ -345,6 +345,11 @@ where
 pub(crate) type StakingCall = crate::Call<Test>;
 pub(crate) type TestCall = <Test as frame_system::Config>::RuntimeCall;
 
+parameter_types! {
+	// if true, skips the try-state for the test running.
+	pub static SkipTryStateCheck: bool = false;
+}
+
 pub struct ExtBuilder {
 	nominate: bool,
 	validator_count: u32,
@@ -454,6 +459,10 @@ impl ExtBuilder {
 		self.balance_factor = factor;
 		self
 	}
+	pub fn try_state(self, enable: bool) -> Self {
+		SkipTryStateCheck::set(!enable);
+		self
+	}
 	fn build(self) -> sp_io::TestExternalities {
 		sp_tracing::try_init_simple();
 		let mut storage = frame_system::GenesisConfig::<Test>::default().build_storage().unwrap();
@@ -582,7 +591,9 @@ impl ExtBuilder {
 		let mut ext = self.build();
 		ext.execute_with(test);
 		ext.execute_with(|| {
-			Staking::do_try_state(System::block_number()).unwrap();
+			if !SkipTryStateCheck::get() {
+				Staking::do_try_state(System::block_number()).unwrap();
+			}
 		});
 	}
 }