Error / Warning marker on empty *.sgen

[BeckmaR]

Our user Pal Csanyi reported to our User Group that he was not able to generate Code from the statechart (Forum post).
We finally figured out he did not select a statechart in the sgen creation wizard, so he ended up with an sgen like this:

GeneratorModel for yakindu::java {	
}

I cannot see any situation where a sgen like this might be useful, so we should consider adding an error / warning marker to sgens that are unable to generate anything, that is:

• completely empty sgens
• sgens with an empty GeneratorModel
  Also, depending on the execution internals of the sgen generator, it might be good to print to the console that no action was done.

Without these informations, it can be hard for beginners to track down why nothing is generated, because they have no idea what an sgen should look like.