Two tiny fixes for Java Vertx client (#3683)

* two tiny fixes:

1.) ApiClient already defines and configures an objectMapper to not fail on unknown properties, but it is not used when parsing the response. The fix uses the pre-configured object mapper instead of the vertx default one

2.) When an operation has no response (or just ones without content), the accept array passed to ApiClient is emtpy. This makes the null check in ApiClient useless, as it still tries to set a null Accept header, which is refused with an NPE. Amend the check with .length > 0 to catch this case.

* update generated client as required by contributor guidelines

modules/openapi-generator/src/main/resources/Java/libraries/vertx/ApiClient.mustache

@@ -444,7 +444,7 @@ public class ApiClient {
 
         updateParamsForAuth(authNames, queryParams, headerParams);
 
-        if (accepts != null) {
+        if (accepts != null && accepts.length > 0) {
             headerParams.add(HttpHeaders.ACCEPT, selectHeaderAccept(accepts));
         }
 
@@ -579,7 +579,7 @@ public class ApiClient {
                             return;
                         } else {
                             try {
-                                resultContent = Json.mapper.readValue(httpResponse.bodyAsString(), returnType);
+                                resultContent = this.objectMapper.readValue(httpResponse.bodyAsString(), returnType);
                                 result = Future.succeededFuture(resultContent);
                             } catch (Exception e) {
                                 result =  ApiException.fail(new DecodeException("Failed to decode:" + e.getMessage(), e));

samples/client/petstore/java/vertx/src/main/java/org/openapitools/client/ApiClient.java

@@ -440,7 +440,7 @@ public <T> void invokeAPI(String path, String method, List<Pair> queryParams, Ob
 
         updateParamsForAuth(authNames, queryParams, headerParams);
 
-        if (accepts != null) {
+        if (accepts != null && accepts.length > 0) {
             headerParams.add(HttpHeaders.ACCEPT, selectHeaderAccept(accepts));
         }
 
@@ -575,7 +575,7 @@ protected <T> Handler<AsyncResult<HttpResponse<Buffer>>> buildResponseHandler(Ty
                             return;
                         } else {
                             try {
-                                resultContent = Json.mapper.readValue(httpResponse.bodyAsString(), returnType);
+                                resultContent = this.objectMapper.readValue(httpResponse.bodyAsString(), returnType);
                                 result = Future.succeededFuture(resultContent);
                             } catch (Exception e) {
                                 result =  ApiException.fail(new DecodeException("Failed to decode:" + e.getMessage(), e));