title: Openapi generator maven plugin nullable property problem
body: <p>I attempted to introduce nullable properties in an API which has been designed using openapi 3 specifications. The idea is to always return the properties to the client, whether their values are null or not.</p>
  
  <p>YAML file (I tried first without default, with same results):</p>
  
  <pre><code>       property:
            type: integer
            nullable: true
            default: null
  </code></pre>
  
  <p>Generated Java code:</p>
  
  <pre><code>  @JsonProperty("property")
    private JsonNullable&lt;Integer&gt; property = JsonNullable.undefined();
  </code></pre>
  
  <p>Response from API:</p>
  
  <pre><code>  "property": {
      "present": true
    }
  </code></pre>
  
  <p>So the result is always "present: true" whether or not the property is null or not. Without nullability it works out just fine, except for the null values being removed from the response which is undesireable.</p>
  
  <p>Any ideas?</p>
  
  <p>P.S. The property isn't actually named as "property"</p>
  
  <p><strong>Edit: configuration:</strong></p>
  
  <pre><code>&lt;generateAliasAsModel&gt;true&lt;/generateAliasAsModel&gt;
  &lt;inputSpec&gt;./api/interface1.yaml&lt;/inputSpec&gt;
  &lt;generatorName&gt;spring&lt;/generatorName&gt;
  &lt;enablePostProcessFile&gt;true&lt;/enablePostProcessFile&gt;
  &lt;configOptions&gt;
      &lt;sourceFolder&gt;src/main/java&lt;/sourceFolder&gt;
      &lt;library&gt;spring-boot&lt;/library&gt;
      &lt;java8&gt;true&lt;/java8&gt;
      &lt;interfaceOnly&gt;true&lt;/interfaceOnly&gt;
      &lt;useOptional&gt;true&lt;/useOptional&gt;
  &lt;/configOptions&gt;
  </code></pre>