Return target_hash for finality_target instead of an Option (#9867)

* .

cargo +nightly fmt --all

* Fix test

* Simplify test

* They are already imported

* Needless clone()

substrate/client/consensus/common/src/longest_chain.rs

@@ -91,11 +91,12 @@ where
 		&self,
 		target_hash: Block::Hash,
 		maybe_max_number: Option<NumberFor<Block>>,
-	) -> Result<Option<Block::Hash>, ConsensusError> {
+	) -> Result<Block::Hash, ConsensusError> {
 		let import_lock = self.backend.get_import_lock();
 		self.backend
 			.blockchain()
 			.best_containing(target_hash, maybe_max_number, import_lock)
+			.map(|maybe_hash| maybe_hash.unwrap_or(target_hash))
 			.map_err(|e| ConsensusError::ChainLookup(e.to_string()).into())
 	}
 }