`map_vec()` fails on empty named lists()

Empty lists `map_vec()` to `NULL`, as expected:

```r
map_vec(list(), identity)
#> NULL
```

But empty *named* lists fail (looks like because they try to restore the names on `NULL`):

```r
map_vec(setNames(list(), character()), identity)
#> Error in names(x) <- names : attempt to set an attribute on NULL
```

Expected behavior: named lists should also just `map_vec()` to `NULL`, right?