Allow zero key length in key and prefix for `DynamoDb`.#4893

MathieuDutSik wants to merge 3 commits intolinera-io:mainlinera-io/linera-protocol:mainfrom

MathieuDutSik:dynamodb_key_accessMathieuDutSik/linera-protocol_second:dynamodb_key_accessCopy head branch name to clipboard