This is a clarification on how PSBT_IN_FINAL_SCRIPTSIG and PSBT_IN_FINAL_SCRIPTWITNESS should be handled when the data is empty. Based on feedback from @achow101 on twitter https://twitter.com/kallerosenbaum/status/1443856371368271872
It seems more people than I had interpreted the bip like you should always set PSBT_IN_FINAL_* even if data is empty, but @achow101 says we should leave those unset.