Fix compilation error with clang 11.0.1.
Building CXX object CMakeFiles/simgrid.dir/src/mc/explo/udpor/maximal_subsets_iterator.cpp.o
In file included from .../src/mc/explo/udpor/maximal_subsets_iterator.cpp:1:
.../src/mc/explo/udpor/maximal_subsets_iterator.hpp:38:3: error: explicitly defaulted default constructor is implicitly deleted [-Werror,-Wdefaulted-function-deleted]
maximal_subsets_iterator() = default;
^
.../src/mc/explo/udpor/maximal_subsets_iterator.hpp:43:68: note: default constructor of 'maximal_subsets_iterator' is implicitly deleted because field 'config' of const-qualified type 'const std::optional<std::reference_wrapper<const Configuration>>' would not be initialized
const std::optional<std::reference_wrapper<const Configuration>> config;
^
1 error generated.
