getConnectableBus() returns exception if there is not bus in voltage level

[miovd]

• Do you want to request a feature or report a bug?
  Bit of both

• What is the current behavior?
  From a node breaker network, if there is a voltage level without bus, getConnectableBus() returns an exception "should not happen".

• What is the expected behavior?
  To be defined: should we accept null if no bus is found? Or should we create a fictitious bus in this case? If so, how do we discriminate fictitious bus from real ones?

• What is the motivation / use case for changing the behavior?
  It makes the import of the CGMES files of some contries to fail.

• Please tell us about your environment:

  • PowSyBl Version: 3.0.0-SNAPSHOT

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

(if a question doesn't apply, you can delete it)