Improve error message for unnest with empty list

[ablaagaard]

Reduced test case:
dataFrameOf("a", "b")(listOf(), 1).unnest("a")
fails with "Provided data does not coerce to tabular shape"

It took me a while to figure out from a larger dataset that the empty list was the problem.
I guess unnest could also be defined to unnest to zero rows in this case, but maybe that is too magical.

[holgerbrandl]

Not sure if unnesting to zero row dataframe is much better here. Do you know how dplyr handles this example?