Unable to store empty strings as values

[fsoikin]

This happens because AWS refuses to encrypt empty payloads:

 λ confcrypt aws add -n FOO -v "" -t String -p -k <key> <file>.econf                                                                                                      

confcrypt: ServiceError (ServiceError' {
  _serviceAbbrev = Abbrev "KMS",
  _serviceStatus = Status {statusCode = 400, statusMessage = "Bad Request"}, 
  _serviceHeaders = [("x-amzn-RequestId","e68c2e4a-791c-4920-97e2-f4778afa963a"),("Cache-Control","no-cache, no-store, must-revalidate, private"),("Expires","0"),("Pragma","no-cache"),("Date","Tue, 19 Jan 2021 17:11:04 GMT"),("Content-Type","application/x-amz-json-1.1"),("Content-Length","175")], 
  _serviceCode = ErrorCode "Validation", 
  _serviceMessage = Just (ErrorMessage "1 validation error detected: Value at 'plaintext' failed to satisfy constraint: Member must have length greater than or equal to 1"), 
  _serviceRequestId = Just (RequestId "e68c2e4a-791c-4920-97e2-f4778afa963a")
})