Fix: GPv3: Add missing check in get_mutable_attribute

The function didn't return early in case the domain was empty.
The fix makes sure we return an empty `MutableSpan` in this case
(add doesn't try to create a new custom data layer).

source/blender/blenkernel/intern/grease_pencil.cc

@@ -266,6 +266,9 @@ 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);