do not write prefix for empty graph id, fix #2154

[eroux]

Summary of changes

Since the names of empty graphs do not appear in the serialization, do not consider them for namespace issues.

fixes #2154

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Added tests for any changes that have a runtime impact.
• Checked that all tests and type checking passes.
• For changes that have a potential impact on users of this project:
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
  • Considered adding an example in ./examples for new features.
  • Considered updating our changelog (CHANGELOG.md).
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[eroux]

Thanks for the merge!

[aucampia]

Sorry for the delay in review, I will try check by the weekend.

[coveralls]

Coverage: 90.63% (-0.004%) from 90.634% when pulling aca39df on eroux:main into 8213b74 on RDFLib:main.

[aucampia]

Change looks good to me. For posterity sake, the reason this addresses the problem is that it avoids TurtleSerializer.getQName which would add the namespace of a each graph in a dataset to the collection of namespaces to serialize.

[eroux]

thanks a lot!