Turn a list of data.frames (of different lengths and potentially empty) into a single indexed data.frame. Useful to process a ResultList from `comprehendHTTP`.
bind_and_index(index, df_list)
Vector of indices
List of data.frames to bind and index. Should NOT be a data.frame.
`index` and `df_list` should be the same length. An error is raised otherwise.
bind_and_index(1:2, list(data.frame(col = "a"), data.frame(col = "b")))
bind_and_index(1:3, list( data.frame(col = "a"), data.frame(), data.frame(c("b", "c"))))