Parsing of empty attributes

While parsing an empty attribute, function parse_node_attributes() correctly creates a new attribute with value of length 0 but it sets the pointer into the buffer instead of to nullptr.
This does not harm in usual circumstances. But in case of cloning, it does. Then allocate_attribute() in clone_node() sets the value as value(val, internal::measure(val)) with length to the end of the buffer which is wrong.

Could you please check?

https://github.com/boostorg/property_tree/blob/29a7f0390b6904e9b7c447d1e6e44442bd0ab17b/include/boost/property_tree/detail/rapidxml.hpp#L2283