[kotlin][spring] Fix ApiUtil compilation (#6084)

Author: sylvainmoindron

modules/openapi-generator/src/main/resources/kotlin-spring/apiUtil.mustache

@@ -12,9 +12,9 @@ object ApiUtil {
     fun setExampleResponse(req: NativeWebRequest, contentType: String, example: String) {
         try {
             val res = req.getNativeResponse(HttpServletResponse::class.java)
-            res.setCharacterEncoding("UTF-8")
-            res.addHeader("Content-Type", contentType)
-            res.getWriter().print(example)
+            res?.characterEncoding = "UTF-8"
+            res?.addHeader("Content-Type", contentType)
+            res?.writer?.print(example)
         } catch (e: IOException) {
             throw RuntimeException(e)
         }

samples/server/petstore/kotlin-springboot-modelMutable/src/main/kotlin/org/openapitools/api/ApiUtil.kt

@@ -9,9 +9,9 @@ object ApiUtil {
     fun setExampleResponse(req: NativeWebRequest, contentType: String, example: String) {
         try {
             val res = req.getNativeResponse(HttpServletResponse::class.java)
-            res.setCharacterEncoding("UTF-8")
-            res.addHeader("Content-Type", contentType)
-            res.getWriter().print(example)
+            res?.characterEncoding = "UTF-8"
+            res?.addHeader("Content-Type", contentType)
+            res?.writer?.print(example)
         } catch (e: IOException) {
             throw RuntimeException(e)
         }

samples/server/petstore/kotlin-springboot/src/main/kotlin/org/openapitools/api/ApiUtil.kt

@@ -9,9 +9,9 @@ object ApiUtil {
     fun setExampleResponse(req: NativeWebRequest, contentType: String, example: String) {
         try {
             val res = req.getNativeResponse(HttpServletResponse::class.java)
-            res.setCharacterEncoding("UTF-8")
-            res.addHeader("Content-Type", contentType)
-            res.getWriter().print(example)
+            res?.characterEncoding = "UTF-8"
+            res?.addHeader("Content-Type", contentType)
+            res?.writer?.print(example)
         } catch (e: IOException) {
             throw RuntimeException(e)
         }