interface: fix S3 canUpload logic
interface: fix S3 canUpload logic

s3.credentials is always set, but the fields are all '' if s3 is not
configured. So prior to this change, if the user had configured an
active bucket, they would always be shown an upload dialog even if they
had no storage configured.

urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,urbit/urbit