Make keystore return None when a key doesn't exist (#8163)

* Make keystore return `None` when a key doesn't exist

* Fixes

* More fixes

* Update comment

* Update primitives/keystore/src/lib.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Update client/keystore/src/local.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

* Address comments

* Update client/keystore/src/local.rs

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

Co-authored-by: André Silva <123550+andresilva@users.noreply.github.com>

substrate/bin/node/cli/src/service.rs

@@ -625,9 +625,7 @@ mod tests {
 					sp_consensus_babe::AuthorityId::ID,
 					&alice.to_public_crypto_pair(),
 					&to_sign,
-				).unwrap()
-				 .try_into()
-				 .unwrap();
+				).unwrap().unwrap().try_into().unwrap();
 				let item = <DigestItem as CompatibleDigestItem>::babe_seal(
 					signature,
 				);