csi: pass nil affinity to ceph-csi-operator when not configured

[ksc98]

Description

When no node affinity is configured, Rook passes an empty Affinity struct to the ceph-csi-operator Driver CR. This prevents the operator from using its own defaults from OperatorConfig, since it only checks for nil (not empty structs).

This PR passes nil instead of an empty struct when no affinity is configured, allowing the ceph-csi-operator to apply its OperatorConfig defaults.

This is the Rook-side fix as suggested in review of ceph/ceph-csi-operator#388 — rather than having the ceph-csi-operator check for empty structs, Rook should simply pass nil when no affinity is configured.

Checklist:

• Commit Message Formatting: Commit titles and messages follow guidelines in the developer guide.
• Reviewed the developer guide on Submitting a Pull Request
• Pending release notes updated with breaking and/or notable changes for the next minor release.
  • Overwriting Ceph's configurations should be marked as breaking changes.
• Documentation has been updated, if necessary.
• Unit tests have been added, if necessary.
• Integration tests have been added, if necessary.