[IIDM v1.0] BBranch calculated connectable bus from NB view can be null

[mathbagu]

• Do you want to request a feature or report a bug?
  Bug fix
  BBranch calculated connectable bus from NB view can be null powsybl-core#1008

• What is the current behavior?
  If no connectableBus is found, an exception is thrown

• What is the expected behavior?
  If no connectableBus is found, no exception is thrown and null is returned

• What is the motivation / use case for changing the behavior?
  Have both Java and C++ API as close as possible

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

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

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