add new encode option method 'encode_empty_table' to control how to encode empty table.

[mythay]

there are three possiable values for the parameter, dict/array/null
If a table is empty, we can control the encoding result to an empty dictionary or empty array of null in json format

[lipp]

👍

[mpx]

I think this would be better handled by a metatable to specify whether an object or table is needed. That would allow more consistent use of types.

[brimworks]

Please see #11

...I agree with @mpx that this needs to be done on a per-value basis rather than a per-encode operation since you may want to mix an empty JSON object with an empty JSON array.