Avoid neutrino related segfault in standalone FoF
When running standalone FoF e->neutrino_properties is a null pointer, which causes a segfault in engine_make_hierarchical_tasks_common() (#773 (closed)). This merge request just adds a check on whether e->neutrino_properties is null. I'm not sure if this is the best solution since it's a bit inconsistent with the way other optional features are treated via engine_policy_*.
In swift itself e->neutrino_properties is always allocated but is zeroed out if we're not using neutrinos.