Improve code coverage of traverse API

[sebalaig]

• Do you want to request a feature or report a bug?
  Potential bug related to !426

• What is the current behavior?
  The code produced by !426 (fix of Stopping traverser #362, Traverser from several vertices #367, Fix bus breaker view traverser #383 (updated in Fix traversers consistency powsybl-core#1844), Fix traversers consistency #387, Update tck traversers unit tests #388, Clarify TraverseResult enum #394) is not fully covered by tests. The following code is not covered :

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem
  N/A

• What is the expected behavior?
  The block should be traversed by tests

• What is the motivation / use case for changing the behavior?
  Improve quality and avoid regressions.

• 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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