[cedar-lean/Protobuf] fix bug parsing empty maps

[cdisselkoen]

Fix incorrect logic when parsing a Proto.Map that has no entries. While we're here, added an HAppend instance for Proto.Map to clean up some calling code slightly

[cdisselkoen]

Hmm this regression test doesn't actually fail without this supposed empty-map "fix". Something else going on here. This "fix" does fix my non-reduced example (an AuthorizationRequest from cedar-drt/protobuf_schema, which unfortunately we aren't set up to easily unit-test at the moment)

[cdisselkoen]

Possible that it only fails on a policyset like this when the LiteralPolicySet is used as an embedded Message in some larger message (like AuthorizationRequest). Which would make this hard to test in a unit test, since none of the types in cedar-policy have protobuf representations that include a LiteralPolicySet as an embedded Message (right?)

[john-h-kastner-aws]

Changes look reasonable, though I don't understand the details of the problems you mention in comments. Does it make sense to merge this as-is given that should fix at least one problem?

[cdisselkoen]

I do think we should merge as-is, even though the added test doesn't catch the problem this is fixing, it is definitely fixing a problem