Ability to bypass attestation verification in sigstore PolicyController

PolicyController is a utility used to enforce supply chain policy in Kubernetes clusters. In versions prior to 0.2.1 PolicyController will report a false positive, resulting in an admission when it should not be admitted when there is at least one attestation with a valid signature and there are NO attestations of the type being verified (--type defaults to "custom"). An example image that can be used to test this is `ghcr.io/distroless/static@sha256:dd7614b5a12bc4d617b223c588b4e0c833402b8f4991fb5702ea83afad1986e2`. Users should upgrade to version 0.2.1 to resolve this issue. There are no workarounds for users unable to upgrade.