`caf::actor_cast` returns non-null-ptr for null-actorhandle

This code:
```
caf::typed_actor<caf::result<void>(caf::get_atom)> actor;
auto ptr = caf::actor_cast<caf::abstract_actor*>(actor);

assert(actor == nullptr);
assert(ptr == nullptr);
```

results in:
```
Assertion `ptr == nullptr' failed.
```

with versions 1.0.2 and 0.19.6