Angus Gibson authored Jul 19, 2022 and Andreas Klöckner committed Jul 19, 2022

A recent change to pybind11 (1d81191, "Disable implicit
conversion of 0 to pybind11::handle.") removes implicit
conversions of "0" to a pybind handle, which the definition of
ISLError relied on. We should explicitly use a nullptr anyway,
which is compatible with newer and older pybind11 versions.