DEPL — Use optional attributes for optional values

Some attributes (like UUIDs and labels) are described as
optional. In the model, optional attributes (multiplicity 0..1)
should be used for them; this has the advantage that no empty
elements are needed in XML documents; optional attributes could
still be mapped to empty (but non-optional, i.e. non-sequence)
strings in the PSM for CCM for IDL.

Proposed resolution:

Make all occurences of "label" and "UUID" attributes optional by
changing "label: String" to "label: String [0..1]", likewise for
UUID. The default value is set to "" (the empty string).

In section 6.4, "PIM for CCM to PSM for CCM for IDL Transformation",
add a paragraph that reads

For all attributes of type String with multiplicity 0..1, update
the multiplicity to 1..1. If the value is missing in an XML re-
presentation of the model, the empty string is used as the default
value.