Handle `.id = NULL` in `ldply`

Per the documentation, of `@param .id`, `NULL` should work as follows: "Pass \code{NULL} to avoid creation of the index column." These changes handle the `NULL` here, though I am sure something in `list_to_dataframe` could work as well. This was the easiest without getting too deep into the guts of the code.

As an aside, the `is.na(.id)` on line 22 gives warning if you pass `NULL`, so I wrote this to check `NULL` first for the short-circuit, and hence no warning. So, if you don't like the way I implemented this, you would want to still do something about that warning because it should not come up.