Follow-up on validating output for (async) actions

> Given the discussion of today's committer meeting:
> - TD spec is not clear about what to expect when the `sync` property is missing
> - Eargerly validate the output in this cases means to try out smart the developer which is quite a bad practice
> - Therefore we decided to merge this PR as a **workaround** for the whole situation. We acknowledge that there needs to be follow up fixes both in Scripting API and Thing Description spec to properly solve the issue.

_Originally posted by @relu91 in https://github.com/eclipse-thingweb/node-wot/pull/1279#pullrequestreview-2104114749_

See also Scripting API issues https://github.com/w3c/wot-scripting-api/issues/554 and https://github.com/w3c/wot-scripting-api/issues/555