hackathon_app: sheets should use local cattr converter
hackathon_app: sheets should use local cattr converter

using the global converter was interfering with looker_sdk: WriteModels
that had None fields (i.e. not set) where getting the value "NA"

looker/sdk-examples,looker-open-source/sdk-examples,looker-open-source/sdk-examples,looker/sdk-examples,looker-open-source/sdk-examples,looker-open-source/sdk-examples,looker/sdk-examples,looker/sdk-examples,looker-open-source/sdk-examples,looker-open-source/sdk-examples,looker/sdk-examples,looker/sdk-examples