[dart-dio] includeIfNull: truefalse bugfix (#17631)

Co-authored-by: Vasiliy Ditsyak <[email protected]>

modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/json_serializable/class.mustache

@@ -43,7 +43,7 @@ class {{{classname}}} {
     {{#defaultValue}}defaultValue: {{{defaultValue}}},{{/defaultValue}}
     name: r'{{{baseName}}}',
     required: {{#required}}true{{/required}}{{^required}}false{{/required}},
-    includeIfNull: {{#required}}{{#isNullable}}true{{/isNullable}}false{{/required}}{{^required}}false{{/required}}
+    includeIfNull: {{#required}}{{#isNullable}}true{{/isNullable}}{{^isNullable}}false{{/isNullable}}{{/required}}{{^required}}false{{/required}}
   )
   {{/isBinary}}
   {{#isBinary}}