Useful in case of boolean values to distinguish between a boolean
value having been set in the native/cross file and not having been
provided, which can't be achieved by passing a fallback parameter
to .get_external_property().
D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082