QVT14 — QVTr: Allow optional Object/CollectionTemplate clauses

An <objectTemplate> may have an empty list of <propertyTemplate> but the surrounding {} are mandatory. Similarly a <collectionTemplate> may have an empty list of members but the surrounding {} are mandatory.

It is common for patterns to refer to properties / members by type and often name without caring about nested properties / members, but because of the syntax the user must contend with a confusing syntax error until the {} are added.

The user experience and patterns can be somewhat simpler if the {} can be omitted for empty content.