Fix #407—list Backblaze S3 contents

[caewok]

Fixes #407. Backblaze S3 will return bucket lists without Owner:DisplayName. This causes as.data.frame.s3_bucket to fail when parsing the contents for get_bucket_df. This PR corrects that issue by ensuring that NULL values are changed to NA in the relevant cases.

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• [ x] if suggesting code changes or improvements, open an issue first
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• [ x] for all but trivial changes (e.g., typo fixes), documentation your change in NEWS.md with a parenthetical reference to the issue number being addressed
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• add code or new test files to /tests for any new functionality or bug fix
• [ x] make sure R CMD check runs without error before submitting the PR