fix: detect nil list items in RNode

[immanuwell]

Fixes a small parser papercut: HasNilEntryInList was walking old map/slice shapes, but RNode stores yaml nodes now. So an empty YAML list item could slip through, not great.

Repro:

go test ./kyaml/yaml -run TestRNodeHasNilEntryInList -count=1

Before this fix, the corrected listWithNil case fails. After it, the path is reported as items/[1] and resource loading rejects the empty item.

Related: #1182

Tested:

go test ./kyaml/yaml ./api/resource -count=1
go test ./kyaml/...

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[immanuwell]

@koba1t @sarab97 please take a look