If it's missing we get an empty value, but we can't use that with
to_string_without_side_effects() so we have to use undefined as the
default.
Fixes#5142.
If we have a function as class extends value, we still cannot assume
that it has a prototype property and that property has a function or
null as its value - blindly calling to_object() on it may fail.
Fixes#5075.
Without this, the oss-fuzz build says:
../Userland/Libraries/LibJS/AST.cpp:58:34: error: member access into incomplete type 'const std::type_info'
return demangle(typeid(*this).name()).substring(4);
^
