Add clarifications about the presence and lack of data schemas

[egekorkan]

We are clear what happens if a data schema, let's say input of actions, exist in a TD or if they don't. For all cases, we should explain the expected behavior when they exist, do not exist or when they are empty (i.e. "input":{}). It should be also clear that these are not optional if the Thing expects the data schema, i.e. input should exist if Thing expects an input payload.