Fix #354: Remove empty RDF Description nodes after unsetModifiedDates

[dyrpsf]

Description

Updated the SBase::reconstructRDFAnnotation() cleanup logic in src/sbml/SBase.cpp. Added a routine at the end of the function to iterate backwards through the RDF node's children and delete any <rdf:Description> elements that have zero inner children. If the parent <rdf:RDF> block becomes completely empty as a result, it is also safely removed.
I also added a unit test (test_SBase_unsetModifiedDates_issue354) in src/sbml/test/TestSBase.cpp to explicitly verify that no detritus is left behind when dates are removed.

Motivation and Context

Previously, calling unsetModifiedDates would successfully remove the date nodes but leave behind empty <rdf:Description rdf:about="#foo"/> elements in the XML tree. Over multiple cycles, these empty tags would accumulate as detritus in the model's annotation. This change ensures the XML tree remains clean after node removal.
Fixes #354

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Change in documentation

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically ```

[fbergmann]

@dyrpsf thank you for this