Allow empty string for protectionPolicyId for it to be unassigned from VolumeGroup

[santhoshatdell]

PR Submission checklist

GitHub Issues

List the GitHub issues impacted by this PR:

GitHub Issue #
dell/csm#928

Common PR Checklist:

• Have you made sure that the code compiles?
• Have you commented your code, particularly in hard-to-understand areas
• Did you run tests in a real Kubernetes cluster?
• Have you maintained backward compatibility

Description of your changes:

• Reverting the changes w.r.t VolumeGroupModify struct that was made during terraform VolumeGroup updates.
• When modify VG call was sent earlier with empty string for protectionPolicyId, it used to unassign the policy from VG so that the VG can be deleted (for replication).
• Tested the array API and noticed no issues there.

Unit tests:
PASS
github.com/dell/gopowerstore coverage: 84.2% of statements
ok github.com/dell/gopowerstore 0.054s coverage: 84.2% of statements
...
PASS
github.com/dell/gopowerstore/api coverage: 81.9% of statements
ok github.com/dell/gopowerstore/api 4.030s coverage: 81.9% of statements