Fixing 2 bugs with IDs

[aaronjeline]

Description of changes

1. Entity types with the id Set were mistakenly interpreted as having the id set
2. A mistake the CFG could lead to a panic when parsing a schema that used in as an entity type name

Issue #, if available

N/A

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• A bug fix or other functionality change requiring a patch to cedar-policy.

I confirm that this PR (choose one, and delete the other options):

• Does not update the CHANGELOG because my change does not significantly impact released code.

I confirm that cedar-spec (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.

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