fix undefined error when duplicates section is not present for workspace item

[tinsch]

References

• Fixes Workspace item duplication check step throws error and causes problems in the frontend DSpace#12865
• Related to Fix bug where workflow item would trigger a nullpointer when checked for versionhistory DSpace#12867

Description

Adds an additional undefined safety check for when the duplicates section is not present in the workspace item json data.

Instructions for Reviewers

1. Have the duplicates section enabled in the submission form
2. Have a duplicate of the item you'd like to create a new version of in a workflow state
3. Try to create a new version of the item as an admin. Observe that you are able to create and edit the new version.

Additional tests

Check that the duplicates section still shows up for unaffected items

Checklist

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LLM Disclosure

I used ChatGPT (Model GPT-5.5 Instant) to help me with the investigation of the bug and to help with the undefined safety check. I fully understand the code and why it fixes the bug. No LLM was used in writing the bug report and the PR text.

[EikLoe]

Hi @tinsch ,

Thanks for the fix. I tested it and it worked fine for me! The code looks good to me, too. So +1

[tdonohue]

👍 Thanks @tinsch ! This looks logically sound to me & I see that @EikLoe tested it.

[dspace-bot]

Successfully created backport PR for dspace-8_x:

• [Port dspace-8_x] fix undefined error when duplicates section is not present for workspace item #6104

[dspace-bot]

Successfully created backport PR for dspace-9_x:

• [Port dspace-9_x] fix undefined error when duplicates section is not present for workspace item #6105

[dspace-bot]

Successfully created backport PR for dspace-10_x:

• [Port dspace-10_x] fix undefined error when duplicates section is not present for workspace item #6106