Referencing non-existing property of empty object passes type check, while non-empty object does not

[anderseknert]

As reported by @disaverio (thanks!) in the OPA Slack.

This is a type checker error

obj := {"foo": "bar}
obj.bar

1 error occurred: policy.rego:5: rego_type_error: undefined ref: obj.bar
	obj.bar
	    ^
	    have: "bar"
	    want (one of): ["foo"]

But this isn't

obj := {"foo": "bar}
obj.bar

I believe the expected behavior would be that both examples fails.