Fix T89302: Broken normals after mesh to curve node

The normals were broken because the normal calculation mode wasn't set.
This patch adds a default normal mode so all code creating a spline does
not necessarily have to set it manually. In the future there should be a
way to change this value in the node tree.

source/blender/blenkernel/BKE_spline.hh

@@ -72,8 +72,7 @@ class Spline {
     Minimum,
     Tangent,
   };
-  /* Only #Zup is supported at the moment. */
-  NormalCalculationMode normal_mode;
+  NormalCalculationMode normal_mode = Minimum;
 
   blender::bke::CustomDataAttributes attributes;