Assignment into empty filter_l

Assigning to a composed `filter_l` + `index_l` generates an error. Seems that assigning into an empty _subset_ and assigning into an empty _dataframe_ are handled differently, e.g.,

```r
# for empty index i
d[i,cols] <- NA  # works
d[i,][,cols] <- NA  # doesn't work
```

Submitted a simple PR — #32 — to perform the assignment only if the set of rows is non-empty.