Fix PutLifecycleConfiguration's Prefix field

[Shunpoco]

Description of your changes

This PR fixes the relationship between Prefix field and Filter field in LifecycleRule.

Currently, if a client want to set a LifecycleConfiguration with Prefix but without Filter, RGW can't take the Prefix field. It causes that the LifecycleConfiguration affects entire objects in the bucket.
Because:

1. RGW first checks if it can decode Filter field, then if not RGW tries if it can decode Prefix (Please see here)
2. In our current GenerateLifecycleRules implementation, even if a client doesn't specify any Filter, the function sets empty LifecycleRuleFilterMemberPrefix instance.
3. Because RGW just checks if it can encode Filter field or not in the if-statement above, the empty LifecycleRuleFilterMemberPrefix instance is encoded and eventually Prefix field is dropped.

I have:

• Run make reviewable to ensure this PR is ready for review.
• Run make ceph-chainsaw to validate these changes against Ceph. This step is not always necessary. However, for changes related to S3 calls it is sensible to validate against an actual Ceph cluster. Localstack is used in our CI Chainsaw suite for convenience and there can be disparity in S3 behaviours betwee it and Ceph. See docs/TESTING.md for information on how to run tests against a Ceph cluster.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

[nolancon]

Nice catch! 🚀