Set sig to Array when empty

[ChevronTango]

right now the tuf init and a few other commands default to setting the sig field to null when it has no valid signatures. A few other commands and CLIs break when they try and read this new json as they are expecting an array, and null is unexpected. Subsequently we have to manually set the sig to [] when writing a script to handle TUF logic so that the next commands will parse the json correctly. Instead the CLI should always return valid JSON with an empty array, rather than a null, so that the downstream JSON parsing always passes correctly.