ValueOnly-Serialization: add information how to deal with optional elements

[BirgitBoss]

What?

in https://github.com/admin-shell-io/submodel-templates/issues/24 the qeustion arises how to deal with optional elements in the serialization.

How?

Add sentence in chap. 11.4.2 ValueOnly-Serialization in JSON (V3.0) Optional submodel elements are omitted if not specified for the submodel.

• [x] I have signed the required Developer Certificate of Origin (DCO) already.

[BirgitBoss]

move to aas-specs because chapter was moved to Part 1