Fix early load exit for unpacked appregistry format

[njhale]

Fixes a load bug that prevents appregistry repos from being entirely loaded.

Tested against community-operators:

$ # load a bunch of packages from community-operators
$ go run ./cmd/appregistry-server/main.go -r https://quay.io/cnr\|community-operators --kubeconfig ~/.kube/config -o microsegmentation-operator,namespace-configuration-operator,microcks,federatorai,spark-gcp,knative-serving-operator,atlasmap-operator,knative-camel-operator,syndesis,knative-kafka-operator,jaeger,descheduler,node-problem-detector,planetscale,hazelcast-enterprise,myvirtualdirectory,kubefed-operator,hco-operatorhub,enmasse,etcd,postgresql,openshift-pipelines-operator,apicurito,open-liberty,kiali,node-network-operator,twistlock,triggermesh,radanalytics-spark,akka-cluster-operator,metering,strimzi-kafka-operator,federation,opsmx-spinnaker-operator,spinnaker-operator,infinispan,grafana-operator,esindex-operator,prometheus,aqua,knative-eventing-operator,ibmcloud-operator,openebs,ripsaw,eclipse-che,iot-simulator,camel-k,cockroachdb,opendatahub-operator,cert-utils-operator,seldon-operator,awss3-operator-registry
... <elided output>
INFO[0018] download complete - 52 repositories have been downloaded  port=50051 type=appregistry
INFO[0019] decoded 50 flattened and 2 nested operator manifest(s)  port=50051 type=appregistry
... <elided output>
INFO[0024] serving registry                              port=50051 type=appregistry

$ # list the set of packages
$ grpcurl -plaintext  localhost:50051 api.Registry/ListPackages
{
  "name": "akka-cluster-operator"
}
... <elided output>
{
  "name": "twistlock"
}

$ # ensure the number of packages matches input (52)
$ grpcurl -plaintext  localhost:50051 api.Registry/ListPackages | rg "name" | wc -l
52

[jpeeler]

nit: the nil slice declaration is the preferred style and works the same way

[jpeeler]

/lgtm

[openshift-ci-robot]

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