Fix for bench usage

Here is a fix that allows to use bench on the ginkgo_integration branch, waiting for a better way to fix this issue (feel free to integrate it or not). In particular, this is why the autotester CI job fails.

Explanation: param_globals::flavor is an openCARP parameter, and is not defined in bench (it is actually a nullptr). This caused the string constructor to fail in fem/slimfem/src/SF_fem_utils.h:2749 when using bench. I added a check on this value so that the PETSc backend is used if this parameter is a null pointer.