Fix: Curves: Missing check in get_mutable_attribute

Same as fd92647f3457bcd8d8c95d93f7e069c97a6599c4.
Without this check, the function would try to create a new
attribute even if `num` was 0.

source/blender/blenkernel/intern/curves_geometry.cc

@@ -244,6 +244,9 @@ static MutableSpan<T> get_mutable_attribute(CurvesGeometry &curves,
                                             const T default_value = T())
 {
   const int num = domain_num(curves, domain);
+  if (num <= 0) {
+    return {};
+  }
   const eCustomDataType type = cpp_type_to_custom_data_type(CPPType::get<T>());
   CustomData &custom_data = domain_custom_data(curves, domain);