Allow management_project_id to be unset

What does this MR do?

When a cluster already has a management_project_id, we can't unset this even though we send params: {management_project_id: ""}.

Add specs for this bug and fixed it.

No changelog necessary as this is a bug introduced and fixed in the same milestone (%12.5)

#32810 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers