top level relationship fix

[zeb33n]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Fixes a bug where the parser would return an empty Document instead of an error if the peer in the top level relationship was not present in the document.

e.g
bom document outline example.spdx now outputs

FATA opening doc: unable to find peer object with SPDXID doesnt-exist 

instead of

               _      
 ___ _ __   __| |_  __
/ __| '_ \ / _` \ \/ /
\__ \ |_) | (_| |>  < 
|___/ .__/ \__,_/_/\_\
    |_|               

 📂 SPDX Document SBOM-SPDX-c6d8b3cc-b41b-4ce9-9e5f-ce24616b7fbe
  │ 
  │ 📦 DESCRIBES 0 Packages
  │ 
  └ 📄 DESCRIBES 0 Files

for the document.

SPDXID: SPDXRef-DOCUMENT

##### Package: nginx

PackageName: nginx
SPDXID: SPDXRef-nginx

##### Package: python 

PackageName: python
SPDXID: SPDXRef-python

Relationship: SPDXRef-nginx CONTAINS SPDXRef-Python
Relationship: SPDXRef-DOCUMENT DESCRIBES doesnt-exist

Note: to repeat this the document needs at least 2 layers of relationships otherwise the elements are added to the document as leaf nodes.

Which issue(s) this PR fixes:

None

None

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Fatal error now encountered when attempting to parse a document with a malformed top level relationship. 

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @zeb33n. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[zeb33n]

hey @cpanato @puerco If you have the time, I would appreciate your feedback here please.