Retrun null

[electriquo]

relates to #250 did not help in this case. how can dasel tell whether a key is set to null?

$ echo '{"foo": null}' | jq '.foo'
null

$ echo '
foo: null
' | yq '.foo'
null

$ echo '{"foo": null}' | dasel -r json '.foo'
Error: could not access map index: property not found: key