OpenAPI: Do not use Optional type as method parameter

In sal-rest-docgen package there is a lot of usages of Optional<> type as method parameter. That’s discouraged: https://stackoverflow.com/questions/31922866/why-should-java-8s-optional-not-be-used-in-arguments.

We can get rid of this problem by slightly reworking our code.

Complete work on https://git.opendaylight.org/gerrit/c/netconf/+/105646 and create additional patches which deals with other occurrences of this problem in sal-rest-docgen package.