Considering the method build_key_path_array, it seems it does not allow to create an empty KeyPathArray:

https://github.com/realm/realm-core/blob/d318021ae7b47a7c38ec14bc9ab10d608c128e72/src/realm/object-store/c_api/notifications.cpp#L67-L74

The method returns std::nullopt in two scenarios:

1. When key_path_array is nullptr.
2. When key_path_array points to an array with a length of zero.

But in case 2 it should return an empty KeyPathArray.