Follow-up on validating output for (async) actions

[danielpeintner]

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 #1279 (review)

See also Scripting API issues w3c/wot-scripting-api#554 and w3c/wot-scripting-api#555