Allow justifications on non-finalized blocks (#1211)

One assertion was unnecessary because of the check right above it,
second assertion resolves
https://github.com/paritytech/polkadot-sdk/issues/1159

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

substrate/client/consensus/grandpa/src/import.rs

@@ -552,7 +552,6 @@ where
 							.into(),
 					))
 				}
-				assert!(block.justifications.is_some());
 				let mut authority_set = self.authority_set.inner_locked();
 				authority_set.authority_set_changes.insert(number);
 				crate::aux_schema::update_authority_set::<Block, _, _>(