fix: add annotation to method parameter

Author: pkernevez

modules/openapi-generator/src/main/resources/JavaSpring/pojo.mustache

@@ -349,7 +349,7 @@ public {{>sealed}}class {{classname}}{{#parent}} extends {{{parent}}}{{/parent}}
    * Convert the given object to string with each line indented by 4 spaces
    * (except the first line).
    */
-  private String toIndentedString(Object o) {
+  private String toIndentedString({{>nullableAnnotation}}Object o) {
     if (o == null) {
       return "null";
     }

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

@@ -5117,6 +5117,7 @@ public void testCollectionTypesWithDefaults_issue_18102() throws IOException {
                 .fileContains("@Nullable Set<String> stringSet")
                 .fileContains("private Set<String> stringDefaultSet = new LinkedHashSet<>(Arrays.asList(\"A\", \"B\"));")
                 .fileContains("private Set<String> stringEmptyDefaultSet = new LinkedHashSet<>();")
+                .fileContains("private String toIndentedString(@Nullable Object o)")
                 .fileDoesNotContain("private List<@Valid TagDto> tags = new ArrayList<>()")
                 .fileDoesNotContain("private Set<@Valid TagDto> tagsUnique = new LinkedHashSet<>()")
                 .fileDoesNotContain("private List<String> stringList = new ArrayList<>()")