Fix #107637: crash when using density brush on empty hair

source/blender/blenkernel/intern/curves_geometry.cc

@@ -338,6 +338,9 @@ Span<int> CurvesGeometry::offsets() const
 }
 MutableSpan<int> CurvesGeometry::offsets_for_write()
 {
+  if (this->curve_num == 0) {
+    return {};
+  }
   implicit_sharing::make_trivial_data_mutable(
       &this->curve_offsets, &this->runtime->curve_offsets_sharing_info, this->curve_num + 1);
   return {this->curve_offsets, this->curve_num + 1};

source/blender/geometry/intern/add_curves_on_mesh.cc

@@ -278,6 +278,9 @@ AddCurvesOnMeshOutputs add_curves_on_mesh(CurvesGeometry &curves,
 
   /* Grow number of curves first, so that the offsets array can be filled. */
   curves.resize(old_points_num, new_curves_num);
+  if (new_curves_num == 0) {
+    return outputs;
+  }
   const IndexRange new_curves_range = curves.curves_range().drop_front(old_curves_num);
 
   /* Compute new curve offsets. */