bug-fix: add check to desc so we dont output null if undefined

[barnjamin]

This seems to choke the js-sdk which is expecting string | undefined

When there is a None in the dict, the json dumped includes null

>>> x = {'z':None}
>>> import json
>>> json.dumps(x)
'{"z": null}'

Other dictify methods have checks for this (Method/Args)

[algochoi]

Thanks for catching this - it looks like we (me) added this check for the method and interface but not for the contract.