[kotlin] Add = null default for optional parameters in jvm-ktor templates and javalin serviceImpls

Expanded test coverage for jvm-ktor optional parameters to verify `= null` defaults in generated code. Updated javalin5 and javalin6 serviceImpl templates to include correct default value assignments.

Author: halotukozak

modules/openapi-generator/src/main/resources/kotlin-server/libraries/javalin5/serviceImpl.mustache

@@ -10,7 +10,7 @@ import kotlinx.coroutines.flow.Flow
 class {{classname}}ServiceImpl : {{classname}}Service {
 {{#operation}}
 
-    override {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{paramName}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^required}}{{^defaultValue}} = null{{/defaultValue}}{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}): {{>returnTypes}} {
+    override {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{paramName}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^-last}}, {{/-last}}{{/allParams}}): {{>returnTypes}} {
         TODO("Implement me")
     }
 {{/operation}}

modules/openapi-generator/src/main/resources/kotlin-server/libraries/javalin6/serviceImpl.mustache

@@ -11,7 +11,7 @@ import kotlinx.coroutines.flow.Flow
 class {{classname}}ServiceImpl : {{classname}}Service {
 {{#operation}}
 
-    override {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{paramName}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^required}}{{^defaultValue}} = null{{/defaultValue}}{{#defaultValue}} = {{{defaultValue}}}{{/defaultValue}}{{/required}}{{^-last}}, {{/-last}}{{/allParams}}{{#hasParams}}, {{/hasParams}}ctx: Context): {{>returnTypes}} {
+    override {{#reactive}}{{^isArray}}suspend {{/isArray}}{{/reactive}}fun {{operationId}}({{#allParams}}{{paramName}}: {{^isBodyParam}}{{>optionalDataType}}{{/isBodyParam}}{{#isBodyParam}}{{^reactive}}{{>optionalDataType}}{{/reactive}}{{#reactive}}{{^isArray}}{{>optionalDataType}}{{/isArray}}{{#isArray}}Flow<{{{baseType}}}>{{/isArray}}{{/reactive}}{{/isBodyParam}}{{^-last}}, {{/-last}}{{/allParams}}{{#hasParams}}, {{/hasParams}}ctx: Context): {{>returnTypes}} {
         TODO("Implement me")
     }
 {{/operation}}

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

@@ -91,6 +91,23 @@ public void testUseResponseAsReturnType(Object useResponseAsReturnType, String e
         assertFileContainsLine(lines, "suspend fun deletePet(@Path(\"petId\") petId: kotlin.Long, @Header(\"api_key\") apiKey: kotlin.String? = null)" + expectedUnitResponse);
     }
 
+    @Test
+    public void testOptionalParamsHaveDefaultNullJvmKtor() throws IOException {
+        OpenAPI openAPI = readOpenAPI("3_0/kotlin/petstore.yaml");
+
+        KotlinClientCodegen codegen = createCodegen(ClientLibrary.JVM_KTOR);
+
+        ClientOptInput input = createClientOptInput(openAPI, codegen);
+
+        DefaultGenerator generator = new DefaultGenerator();
+        enableOnlyApiGeneration(generator);
+
+        List<File> files = generator.opts(input).generate();
+        File petApi = files.stream().filter(file -> file.getName().equals("PetApi.kt")).findAny().orElseThrow();
+
+        assertFileContains(petApi.toPath(), "apiKey: kotlin.String? = null");
+    }
+
     @Test
     public void testEnumDefaultForReferencedSchemaParameterJvmOkhttp4() throws IOException {
         OpenAPI openAPI = readOpenAPI("3_0/kotlin/enum-default-query.yaml");