Add helper for clearing blank model attributes before validation

What does this MR do?

Create helper to set blank attributes to nil before validation

Uses a before_validation callback to set configured attributes to nil if they have a truthy response to #blank?, which helps prevent empty strings ending up in the database.

As a proof of concept, adds a use of the helper where we previously did this manually (Clusters::Platforms::Kubernetes#namespace)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done