🐛 Don't crash when outputting an empty path object

Previously, applying `str` or `repr` to an object representing the
path `<path d="">` would abort with an AttributeError because the
bounding box is None, which does not support the `center` method.
Now, we use (NaN, NaN) as the center of an empty path.

simpinkscr/simple_inkscape_scripting.py

@@ -472,6 +472,10 @@ def __repr__(self):
         'Return a unique description of the object as a string.'
         iobj = self.get_inkex_object()
         bbox = iobj.bounding_box()
+        if bbox is None:
+            # Make empty paths produce a center of (NaN, NaN) instead
+            # of crashing with an AttributeError.
+            bbox = inkex.BoundingBox()
         return '<%s %s (%s) %s>' % \
             (self.__class__.__name__,
              iobj.TAG,