Null network in diagram style provider

[flo-dup]

• Do you want to request a feature or report a bug?
  Bug

• What is the current behavior?
  Network can be null in AbstractBaseVoltageDiagramStyleProvider, leading to NullPointerExceptions.

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

  • Provide a null network in TopologicalStyleProvider or NominalVoltageDiagramStyleProvider
  • Do not disable highlightLineState in LayoutParameters
  • Enable addNodesInfos: LayoutParameters.setAddNodesInfos(true)

• What is the expected behavior?
  No NPE. Or exception when creating DiagramStyleProvider, but I think some users which don't handle a network still need a NominalVoltageDiagramStyleProvider (see this spectrum question).

• What is the motivation / use case for changing the behavior?
  Avoid NPE.