fix: Resolve issue with empty node set (#1744)

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## Description
Transform default node set value needed for FastAPI to default value
needed by Cognee

## Type of Change
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- [x] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Code refactoring
- [ ] Performance improvement
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## Pre-submission Checklist
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issue/feature**
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cognee/api/v1/add/routers/get_add_router.py

@@ -82,7 +82,9 @@ def get_add_router() -> APIRouter:
                 datasetName,
                 user=user,
                 dataset_id=datasetId,
-                node_set=node_set if node_set else None,
+                node_set=node_set
+                if node_set != [""]
+                else None,  # Transform default node_set endpoint value to None
             )
 
             if isinstance(add_run, PipelineRunErrored):