[FRAME] Test for sane genesis default (#3412)

Closes https://github.com/paritytech/polkadot-sdk/issues/2713

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

substrate/frame/session/src/lib.rs

@@ -460,27 +460,13 @@ pub mod pallet {
 					);
 					self.keys.iter().map(|x| x.1.clone()).collect()
 				});
-			assert!(
-				!initial_validators_0.is_empty(),
-				"Empty validator set for session 0 in genesis block!"
-			);
 
 			let initial_validators_1 = T::SessionManager::new_session_genesis(1)
 				.unwrap_or_else(|| initial_validators_0.clone());
-			assert!(
-				!initial_validators_1.is_empty(),
-				"Empty validator set for session 1 in genesis block!"
-			);
 
 			let queued_keys: Vec<_> = initial_validators_1
-				.iter()
-				.cloned()
-				.map(|v| {
-					(
-						v.clone(),
-						Pallet::<T>::load_keys(&v).expect("Validator in session 1 missing keys!"),
-					)
-				})
+				.into_iter()
+				.filter_map(|v| Pallet::<T>::load_keys(&v).map(|k| (v, k)))
 				.collect();
 
 			// Tell everyone about the genesis session keys