Fix: crash duplicating empty hair

Attempt to access `points_by_curve.data().last()` on an empty array
crashed on a null pointer dereference.

Ref !135956

source/blender/editors/curves/intern/curves_edit.cc

@@ -116,6 +116,9 @@ static void curve_offsets_from_selection(const Span<IndexRange> selected_points,
 
 void duplicate_points(bke::CurvesGeometry &curves, const IndexMask &mask)
 {
+  if (curves.is_empty()) {
+    return;
+  }
   const OffsetIndices<int> points_by_curve = curves.points_by_curve();
   const VArray<bool> src_cyclic = curves.cyclic();