Fix assignment into empty filter lens

[arbelt]

Assigning into an empty filter doesn't work when composed with an index_l afterwards. This seems to have to do with special semantics in data.frame indexing, e.g.,

d[i,"c"] <- y

works when i is empty, but

d[i,][,"c"] <- y

does not. I've added a simple check to ensure that the filter is not empty before performing the assignment.