Storage chain fixes + guide (#9504)

* Transaction storage guide and fixes

* Apply suggestions from code review

Co-authored-by: Bruno Škvorc <bruno@skvorc.me>

* Update frame/transaction-storage/README.md

Co-authored-by: Bruno Škvorc <bruno@skvorc.me>

* Extended example

Co-authored-by: Bruno Škvorc <bruno@skvorc.me>

substrate/primitives/transaction-storage-proof/src/lib.rs

@@ -164,8 +164,9 @@ pub mod registration {
 		}
 
 		let proof = match client.block_indexed_body(number)? {
-			Some(transactions) => Some(build_proof(parent.as_ref(), transactions)?),
-			None => {
+			Some(transactions) if !transactions.is_empty() =>
+				Some(build_proof(parent.as_ref(), transactions)?),
+			Some(_) | None => {
 				// Nothing was indexed in that block.
 				None
 			},