title: WebClient's bodyToMono on Empty Body Expected Behavior
body: <p>What is the expected behavior when WebClient bodyToMono encounters an empty body? In my specific example we are checking the status returned from a post call, and if it's an error converting it to our custom error format. If the conversion to the custom error format fails, we create a new error in our custom format stating that. But when a response came in that was an error with an empty body, it failed to send any error back at all because bodyToMono didn't fail as I had expected. See the below code block:</p>
  
  <pre><code>.retrieve()
  .onStatus(HttpStatus::isError) { response -&gt;
      response.bodyToMono(ErrorResponse::class.java)
          .doOnError {
              throw APIException(
                  code = UNEXPECTED_RESPONSE_CODE,
                  reason = it.message ?: "Could not parse error response from Inventory Availability",
                  httpStatus = response.statusCode()
              )
          }
          .map {
              throw APIException(
                  reason = it.errors.reason,
                  code = it.errors.code,
                  httpStatus = response.statusCode()
              )
          }
  }
  </code></pre>
  
  <p>To fix this we added the switchIfEmpty. </p>
  
  <pre><code>.retrieve()
  .onStatus(HttpStatus::isError) { response -&gt;
      response.bodyToMono(ErrorResponse::class.java)
          .switchIfEmpty { throw RuntimeException("Received Empty Response Body") }
          .doOnError {
              throw APIException(
                  code = UNEXPECTED_RESPONSE_CODE,
                  reason = it.message ?: "Could not parse error response from Inventory Availability",
                  httpStatus = response.statusCode()
              )
          }
          .map {
              throw APIException(
                  reason = it.errors.reason,
                  code = it.errors.code,
                  httpStatus = response.statusCode()
              )
          }
  }
  </code></pre>
  
  <p>My question is this: Is this expected behavior from bodyToMono? Since I'm explicitly asking to map the response body to my object of ErrorResponse, I would expect an empty body to error and then hit the doOnError block, but instead it just "succeeds"/returns an empty mono...hence us adding the switchIfEmpty block. If the object we were mapping to had nullable fields, I could see it not having an error, but since all fields are required why does bodyToMono not throw an error when trying to map "nothing" to my object?</p>