[Java][native] Fix: empty deepObject can produce invalid query [fix #18736] (#18737)

* fix empty query parameter string

* update samples

Author: marcel-huber-ptv

modules/openapi-generator/src/main/resources/Java/libraries/native/api.mustache

@@ -372,7 +372,10 @@ public class {{classname}} {
       }
             {{/isArray}}
             {{^isArray}}
-      localVarQueryStringJoiner.add({{paramName}}.toUrlQueryString("{{baseName}}"));
+      String queryString = {{paramName}}.toUrlQueryString("{{baseName}}");
+      if (!queryString.isBlank()) {
+        localVarQueryStringJoiner.add(queryString);
+      }
             {{/isArray}}
     }
         {{/isDeepObject}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientDeepObjectTest.java

@@ -59,6 +59,6 @@ public void deepObject() throws IOException {
 
         assertFileContains(Paths.get(outputPath + "/src/main/java/org/openapitools/client/api/DefaultApi.java"),
                 "options.toUrlQueryString(\"options\")",
-                "inputOptions.toUrlQueryString(\"inputOptions\"))");
+                "inputOptions.toUrlQueryString(\"inputOptions\")");
     }
 }

samples/client/echo_api/java/native/src/main/java/org/openapitools/client/api/QueryApi.java

@@ -453,7 +453,10 @@ private HttpRequest.Builder testQueryStyleDeepObjectExplodeTrueObjectRequestBuil
     String localVarQueryParameterBaseName;
     localVarQueryParameterBaseName = "query_object";
     if (queryObject != null) {
-      localVarQueryStringJoiner.add(queryObject.toUrlQueryString("query_object"));
+      String queryString = queryObject.toUrlQueryString("query_object");
+      if (!queryString.isBlank()) {
+        localVarQueryStringJoiner.add(queryString);
+      }
     }
 
     if (!localVarQueryParams.isEmpty() || localVarQueryStringJoiner.length() != 0) {
@@ -546,7 +549,10 @@ private HttpRequest.Builder testQueryStyleDeepObjectExplodeTrueObjectAllOfReques
     String localVarQueryParameterBaseName;
     localVarQueryParameterBaseName = "query_object";
     if (queryObject != null) {
-      localVarQueryStringJoiner.add(queryObject.toUrlQueryString("query_object"));
+      String queryString = queryObject.toUrlQueryString("query_object");
+      if (!queryString.isBlank()) {
+        localVarQueryStringJoiner.add(queryString);
+      }
     }
 
     if (!localVarQueryParams.isEmpty() || localVarQueryStringJoiner.length() != 0) {