Handle empty key in Utils.checkThatBucketHasObjectLifecycleConfiguration

[JoshRosen]

This patch guards against key being null in Utils.checkThatBucketHasObjectLifecycleConfiguration, which can happen if tempdir is configured to point to the root of a S3 bucket.

Fixes #91.

[codecov-io]

Current coverage is 94.84%

Merging #95 into master will not affect coverage as of b224b9b

@@            master     #95   diff @@
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  Files           11      11       
  Stmts          446     446       
  Branches       109     109       
  Methods          0       0       
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  Hit            423     423       
  Partial          0       0       
  Missed          23      23       

Review entire Coverage Diff as of b224b9b

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