Curves: Avoid unnecessarily initializing new positions layer

When creating a curves data-block, one is expected to set the new
position values. We can slightly improve performance by avoiding
doing that redundantly.

Similar to cccc6d6905.

source/blender/blenkernel/intern/curves_geometry.cc

@@ -58,7 +58,7 @@ CurvesGeometry::CurvesGeometry(const int point_num, const int curve_num)
 
   CustomData_add_layer_named(&this->point_data,
                              CD_PROP_FLOAT3,
-                             CD_SET_DEFAULT,
+                             CD_CONSTRUCT,
                              nullptr,
                              this->point_num,
                              ATTR_POSITION.c_str());