[R][Client] Fix api response, JSON for maps and let httr2 api client handle empty response bodies

[joXemMx]

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ApiResponse fix:
The current code for response_to_text is:

response_as_text = function(from_encoding = NULL, to_encoding = "UTF-8") {
      text_response <- iconv(readBin(self$response, character()), from = from_encoding, to = to_encoding)
      if (is.na(text_response)) {
        warning("The response is binary and will not be converted to text.")
      }
      return(text_response)
    }

But from_encoding = NULL is invalid and produces an error, as only strings are allowed, see iconv docs https://www.rdocumentation.org/packages/base/versions/3.6.2/topics/iconv. I changed it to an empty string, which works as intended:

response_as_text = function(from_encoding = "", to_encoding = "UTF-8") {
      text_response <- iconv(readBin(self$response, character()), from = from_encoding, to = to_encoding)
      if (is.na(text_response)) {
        warning("The response is binary and will not be converted to text.")
      }
      return(text_response)
    }

JSON for maps fix:
Currently maps are generated like

"supportedILPSolvers": "{
        "CLP": "2.10.8.2",
        "GUROBI": "9.1.1",
        "CPLEX": "12.7.1"
    }"

by code following this pattern (actual snippet of my generated code):

if (!is.null(self$`supportedILPSolvers`)) {
          sprintf(
          '"supportedILPSolvers":
            "%s"
          ',
          jsonlite::toJSON(lapply(self$`supportedILPSolvers`, function(x){ x }), auto_unbox = TRUE, digits = NA)
          )
        }

which is wrong JSON and produces an error in the classes toJSONString function.
I simply changed the template to produce %s instead of "%s" for maps, which results in perfect JSON and removes the error. Resulting JSONs look like

"supportedILPSolvers": {
        "CLP": "2.10.8.2",
        "GUROBI": "9.1.1",
        "CPLEX": "12.7.1"
    }

httr2 api_client handling empty response bodies:
As it is currently, an empty response body will lead to

Backtrace:
    ▆
 1. ├─api_instance$CloseProjectSpace(project_id)
 2. │ └─self$CloseProjectSpaceWithHttpInfo(project_id, ...) at generated/R/projects_api.R:492:6
 3. │   └─self$api_client$CallApi(...) at generated/R/projects_api.R:539:6
 4. │     └─self$Execute(...) at generated/R/api_client.R:153:6
 5. │       └─resp %>% resp_body_raw() at generated/R/api_client.R:263:8
 6. └─httr2::resp_body_raw(.)
 7.   └─rlang::abort("Can not retrieve empty body")

in the httr2 api_client.R. I simply rewrote this to take the value of NULL instead of calling resp %>% resp_body_raw() when the response body is empty.

Mentioning technical staff: @Ramanth @saigiridhar21

[wing328]

thanks for the PR. for this change, can we add a test model in the test spec to ensure it's tested moving forward?

[joXemMx]

Please clarify what you need, or link a PR that did something similar. I would be glad to contribute some testing!

[wing328]

please update modules/openapi-generator/src/test/resources/3_0/r/petstore.yaml with test schemas (models) to ensure the above change works as expected (and then regenerate the samples)

[joXemMx]

I added a minimal map schema that leads to the generation of the toJSONString behavior I mentioned above. Please see if this is what you wanted.

[wing328]

lgtm. lets give it a try