add omitempty tag to ViewFilter in ViewFilterDefinition

[achugh-sumo]

Description

Fix: Add omitempty tag to ViewFilter Problem: Creating Sumo Logic roles with selection_type = "Deny" fails with "view_filter_should_be_null" error when view_filter is not specified.

Cause: Unset view_filter fields were serialized as empty strings ("") instead of being omitted from JSON, causing backend validation to fail.

Fix: Added omitempty JSON tag to ViewFilterDefinition.ViewFilter field. Empty values are now omitted from the JSON payload.

Result: Users can now create and update Deny roles without validation errors.

Check list

• [x] Describe the changes and their purpose
• [ ] Update HTML docs (if applicable)
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• [x] Update CHANGELOG.md
• [ ] Check CONTRIBUTING.md for anything forgotten