A 200 response with `result: 1` but no `epidata` returns a silent empty tibble — inconsistent with the `result: -2` contract

[docxology]

Summary

While following the envelope handling, I noticed an asymmetry: {"epidata": [], "result": -2, "message": "no results"} is treated as an error/warning path (the contract is even pinned by a test), but a 200 body of {"result": 1, "message": "success"} with no epidata field at all flows straight through: response_content$epidata is NULL, parse_data_frame() produces a 0×0 tibble, and fetch() returns it with no error or warning. A malformed envelope masquerades as a legitimate empty result, so users may conclude "no data exists" when the response was actually broken. I think the fix is to require a non-NULL epidata after the envelope check.

Evidence (dev tip 87b10e1)

R/epidatacall.R:430-432:

# classic: JSON with result/message wrapper
check_epidata_result(response_content, allow_empty = fetch_args$return_empty)
return(response_content$epidata)     # NULL when 'epidata' is absent

parse_data_frame() handles it without complaint (R/model.R:268-272): df <- as.data.frame(NULL) → 0×0 tibble. An object-shaped epidata ({...} instead of an array) becomes a 1-row tibble of untyped columns.

Contrast — the -2 contract is pinned in tests/testthat/test-cache.R:111-114:

empty_fixture <- to_httr2_response('{"epidata":[],"result":-2,"message":"no results"}')
...
expect_warning(empty_call <- epidata_call %>% fetch())

Maintainer-runnable repro

Inside a testthat context using the package's existing mock helper:

epidata_call <- pvt_cdc(auth = "k", locations = "ma", epiweeks = epirange(202003, 202304))
resp <- to_httr2_response('{"result":1,"message":"success"}')   # no 'epidata'
testthat::local_mocked_bindings(req_perform = function(req, ...) resp, .package = "httr2")
fetch(epidata_call)   # returns a 0x0 tibble — no error, no warning

Expected vs. actual

• Expected: result: 1 without epidata is a malformed envelope → error (or at least a warning with a malformed-envelope class), consistent with how the package treats the equally data-less -2 body.
• Actual: silent 0×0 tibble; downstream code cannot distinguish "legitimately empty" from "broken response".

Suggested fix

After the envelope check, require the field:

if (is.null(response_content$epidata)) {
  cli::cli_abort(
    "malformed response envelope: 'result' is 1 but 'epidata' is missing",
    class = "epidatr__malformed_envelope"
  )
}

and verify the value is an array (list/data.frame) before parse_data_frame().

I checked for prior reports (open and closed issues/PRs about empty results — #356/#354/#364 cover cast-API empty handling, not the classic envelope) and found none covering this.