Fix: Python: Incorrect default curve_type when calling drawing.add_strokes()

The API did not initialize the `curve_type` for the newly created strokes resulting in unwanted behavior.

Set the curve type to the expected default: `POLY`.

source/blender/makesrna/intern/rna_grease_pencil_api.cc

@@ -25,6 +25,7 @@ const EnumPropertyItem rna_enum_tree_node_move_type_items[] = {
 
 #  include "BKE_attribute.hh"
 #  include "BKE_context.hh"
+#  include "BKE_curves.hh"
 #  include "BKE_grease_pencil.hh"
 #  include "BKE_report.hh"
 
@@ -45,6 +46,10 @@ static void rna_GreasePencilDrawing_add_curves(ID *grease_pencil_id,
     return;
   }
 
+  /* Default to `POLY` curves for the newly added ones. */
+  drawing.strokes_for_write().curve_types_for_write().take_back(sizes_num).fill(CURVE_TYPE_POLY);
+  drawing.strokes_for_write().update_curve_types();
+
   drawing.tag_topology_changed();
 
   /* Avoid updates for importers. */