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fix(kyaml/yaml): minor nil safety fix for RNode.Content etc

fix(kyaml/yaml): minor nil safety fix for RNode.Content etc: a task in rh-swe-bench (Harbor dataset). Title: fix(kyaml/yaml): minor nil safety fix for RNode.Content etc

The task

**Title:** fix(kyaml/yaml): minor nil safety fix for RNode.Content etc

Part of rounakbende/rh-swe-bench.