Allow to override the default behavior in case of
bad layers (which is to invalidate the whole project).
When set to TRUE, the bad layers are skipped and the
project is considered available.
Followup #8922
For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!