[kotlin][spring] Fix ApiUtil compilation

Created by: sylvainmoindron

fixes #5476 (closed) The kotlin-spring generator ApiUtil object doesn't compile, by adding null safe accessor

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• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
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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/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-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)
         }