clientPostRenderCallback can be nil

[alehander92]

I tried new karax with existing code, and it seems setRenderer f leads to errors, as wrapPostRender tries to call nil.

[Araq]

It doesn't anymore:

  proc wrapPostRender(data: RouterData) =
    if clientPostRenderCallback != nil: clientPostRenderCallback()
  setRenderer(wrapRenderer, root, wrapPostRender)