Fix error for name of newly created feature model

[kbirken]

The root feature of a newly created feature model deliberately lacks a name, thus an error is shown. This is correct.

However, the name of the feature model itself is derived from the root feature's name via a get-property constraint. As both the feature model concept and the root feature concept are implementing IValidNamedConcept, the non-existent name of a new root feature will cause a second error for the name of the feature model. This second error will not vanish automatically after some valid name is entered, only after F5 is pressed additionally.

This PR fixes the problem by assigning a specific NO_NAME name to feature models without root feature or with root features with an invalid name.

This solves #1713.

[kbirken]

Regarding @dbinkele 's offline question about using StringUtils.isEmpty vs. StringUtils.isBlank: The condition should apply specifically when the feature model is created. As soon as the user enters one or more spaces themselves, it is their responsibility, and the error message may then be displayed at the feature model. So we should use isEmpty rather than isBlank.