call DeletePolicy if bucket policy is empty

[michaelhtm]

Issue #2731

Description of changes:
currently, we call DeletePolicy is not nil. This causes issues if the
value of the bucket policy is an empty string.
With these changes, we call DeletePolicy when the bucket policy is nil
or empty.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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[ack-prow]

@michaelhtm: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
s3-verify-code-gen	77bbf47	link	false	/test s3-verify-code-gen

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[michaelhtm]

/test s3-kind-e2e