Adding the useOptional in the ApiController (#6735)

* Adding the useOptional in the ApiController

* If jdk8 is disabled but useOptional is enabled, avoid duplicate import.

Author: LionH

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

@@ -37,6 +37,11 @@ import javax.validation.Valid;
 {{#jdk8}}
 import java.util.Optional;
 {{/jdk8}}
+{{^jdk8}}
+    {{#useOptional}}
+import java.util.Optional;
+    {{/useOptional}}
+{{/jdk8}}
 {{^jdk8}}
 import java.util.List;
 import java.util.Map;