guac: [ingestion bug] Failure due to missing purl attribute

Describe the bug A clear and concise description of what the bug is.

We tried to ingest sbom files (both CDX and SPDX) using the latest guacone binary (on the main branch) however it errors out for a few files. Following are the logs for the same.

"msg":"emit error: unable to assemble graphs: Node {<image-name>[sha256:95a7593269fc3344af31c8825e72da6f705dd753417581dd0391e0296ed81242] sha256:95a7593269fc3344af31c8825e72da6f705dd753417581dd0391e0296ed81242 [] [container] {file:///<file-absolute-path>.cdx.json FileCollector}} has no value for property purl","stacktrace":"github.com/guacsec/guac/pkg/handler/collector.Collect\n\t/Users/smenezes/dev/guac-exp/guac/pkg/handler/collector/collector.go:100\ngithub.com/guacsec/guac/cmd/guacone/cmd.glob..func5\n\t/Users/smenezes/dev/guac-exp/guac/cmd/guacone/cmd/files.go:169\ngithub.com/spf13/cobra.(*Command).execute\n\t/Users/smenezes/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:920\ngithub.com/spf13/cobra.(*Command).ExecuteC\n\t/Users/smenezes/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:1044\ngithub.com/spf13/cobra.(*Command).Execute\n\t/Users/smenezes/go/pkg/mod/github.com/spf13/cobra@v1.6.1/command.go:968\ngithub.com/guacsec/guac/cmd/guacone/cmd.Execute\n\t/Users/smenezes/dev/guac-exp/guac/cmd/guacone/cmd/root.go:104\nmain.main\n\t/Users/smenezes/dev/guac-exp/guac/cmd/guacone/main.go:23\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:250"}

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GUAC version Are you using a released version of GUAC or one build from a commit in the repository? The one on the main branch.

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