Fix: NFT package JSON encoding

[taramakage]

1. On the Sending Chain side, unused optional fields won't show up in JSON
   1. if classUri/classData == "", omitted.
   2. if tokenUris/tokenData == [] or ["","",""], omitted.
   3. if tokenUris/tokenData == ["a","b",""] or ["a","b", "c"], show up as it is.
2. On the Receiving Chain side, unused fields will be converted into pkg struct as below:
   1. field omitted will be converted as "" for string or ["","",""] for string array.
   2. "classUri/classData": null will be converted as empty string ""
   3. "tokenUris/tokenData": ["1", null, "3"] will be converted as ["1", "", "3"]