When importing a .x3d file containing an empty IndexedFaceSet, like this for example:

Shape {
  geometry IndexedFaceSet {
    coord Coordinate {
      point [

      ]
    }
    coordIndex [

    ]
  }
}

import_x3d.py throws an exception, because x_min, x_max, ... are not initialized/still set to None.
This fixes the issue by initializing the mentioned variables to inf/-inf respectively and also further simplifies the code by utlizing the min and max builtins.