[R] Handle conversion of extra nested struct column

[asfimport]

Followup to ARROW-10570. See #8650

Reporter: Neal Richardson / @nealrichardson
Assignee: Romain Francois / @romainfrancois

PRs and other links:

• GitHub Pull Request #9919

_{Note: This issue was originally created as ARROW-11832. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Issue resolved by pull request 9919
#9919