deep in terraform it looks like we have:

{
  "Code": {
    "ZipFile": "UEsFBgAAAAAAAAAAAAAAAAAAAAAAAA=="
  },
  "Description": "",
  "FunctionName": "docsHandlerFunc-dfeb6fb",
  "Handler": "",
  "MemorySize": 128,
  "PackageType": "Zip",
  "Publish": false,
  "Role": "arn:aws:iam::XXXXXXXX:role/docsHandlerRole-87ca993",
  "Runtime": "nodejs12.x",
  "Timeout": 3
}

which seems to map to https://docs.aws.amazon.com/lambda/latest/dg/API_CreateFunction.html

ok, looks like according to the docs handler is expected to be [^\s]+, so "" is invalid

specifying a dummy handler name gets me to a different error

so yay

ok, this was my problem (I'm running a custom runtime so handler doesn't matter).

will file a bug/patch later today