changes output of ncbi_children when there are no children

[zachary-foster]

When there are no children for a given taxon, NULL was being returned. This is fine when out_type = "uid", sense length(NULL) = 0 so something like sapply(ncbi_children_output, length) will give the expected results. However, if out_type = "summary" and a list of data.frames are the expected output, sapply(ncbi_children_output, nrow) will have NULLs where 0 would logically be. I changed it so that when out_type = "summary", taxa with no children are represented by a data.frame with no rows, thus producing the expected result.

[coveralls]

Changes Unknown when pulling 569c8af on zachary-foster:master into * on ropensci:master*.

[sckott]

looks good, thanks @zachary-foster