Use tidyselect allow_empty

[olivroy]

Just testing if my PR in tidyselect seems to work as expected.

Not to be merged.

Edit: the genuine snapshot failures (caused by the addition of a classed error) actually nods that this change in tidyselect would be useful for tidyr.

For example

    (expect_error(pivot_wider(df, names_from = key, values_from = starts_with("foo")))
    )
  Output
-   <error/rlang_error>
+   <error/tidyselect:::error_disallowed_empty>
    Error in `pivot_wider()`:
    ! Must select at least one item.

Could be improved if the eval_select() call had error_arg = "foo"