Make system name mandatory for classification requirement

According to [this](https://github.com/buildingSMART/IDS/blob/master/Documentation/testcases/classification/fail-a_classification_facet_with_no_data_matches_any_classification_1_2.ids) example it is possible to create a classification requirement that is accepted if an object has any classification. That is, it is possible to have this kind of requirement:
```



```

My interpretation of this is that if an object happens to have exactly one classification of any kind, the requirement is met. I don't see that this is a feasible case. I'd propose to make the system obligatory in classification.