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MINOR: [R] Fix for dev purrr (#14581)

The recycling rules in map2() are now stricter, so we need to check that `x` actually has columns before applying the metadata.

I also mildly refactored the test to make it easier to run in isolation; I'm happy to revert those changes if desired.

Authored-by: Hadley Wickham <h.wickham@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>