fix(jaxrs): PR comment

Author: Dominik Puk

modules/openapi-generator/src/main/resources/JavaJaxRS/spec/pojo.mustache

@@ -174,7 +174,7 @@ public class {{classname}} {{#parent}}extends {{{.}}}{{/parent}} {{#vendorExtens
   {{#isArray}}
   public {{classname}} add{{nameInPascalCase}}Item({{{items.datatypeWithEnum}}} {{name}}Item) {
     {{#vendorExtensions.x-is-jackson-optional-nullable}}
-    if (this.{{name}} == null || !this.{{name}}.isPresent()) {
+    if (this.{{name}} == null || !this.{{name}}.isPresent() || this.{{name}}.get() == null) {
       this.{{name}} = JsonNullable.<{{{datatypeWithEnum}}}>of({{{defaultValue}}}{{^defaultValue}}new {{#uniqueItems}}LinkedHashSet{{/uniqueItems}}{{^uniqueItems}}ArrayList{{/uniqueItems}}<>(){{/defaultValue}});
     }
     try {
@@ -216,7 +216,7 @@ public class {{classname}} {{#parent}}extends {{{.}}}{{/parent}} {{#vendorExtens
   {{#isMap}}
   public {{classname}} put{{nameInPascalCase}}Item(String key, {{{items.datatypeWithEnum}}} {{name}}Item) {
     {{#vendorExtensions.x-is-jackson-optional-nullable}}
-    if (this.{{name}} == null || !this.{{name}}.isPresent()) {
+    if (this.{{name}} == null || !this.{{name}}.isPresent() || this.{{name}}.get() == null) {
       this.{{name}} = JsonNullable.<{{{datatypeWithEnum}}}>of({{{defaultValue}}}{{^defaultValue}}new HashMap<>(){{/defaultValue}});
     }
     try {