fix: clear policy exclusions to avoid JSON serialization errors

When retrieving a policy from the API and updating it, the exclusions
may contain null scope values that cause Gson serialization errors:
"Not a JSON Object: null" at StorageScope serialization.

This fix clears the exclusions array before updating the policy to
avoid the serialization issue while still allowing the tests to
modify enforcement actions and policy fields.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: janisz

functionaltest-jenkins-plugin/src/test/groovy/ImageScanningTest.groovy

@@ -112,6 +112,8 @@ class ImageScanningTest extends BaseSpecification {
         policy.setEnforcementActions(enforcements)
         policy.setFields(new StoragePolicyFields().imageName(new StorageImageNamePolicy().tag(tag)))
         policy.setDisabled(false)
+        // Clear exclusions to avoid serialization issues with null scope values
+        policy.setExclusions([])
         restApiClient.updatePolicy(policy, policyId)
         return restApiClient.getPolicy(policyId)
     }