core: Don't draw None edge labels (#24690)

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- [ ] **PR title**: "package: description"
- Where "package" is whichever of langchain, community, core,
experimental, etc. is being modified. Use "docs: ..." for purely docs
changes, "templates: ..." for template changes, "infra: ..." for CI
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- [ ] **PR message**: ***Delete this entire checklist*** and replace
with
    - **Description:** a description of the change
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mention, we'll gladly shout you out!


- [ ] **Add tests and docs**: If you're adding a new integration, please
include
1. a test for the integration, preferably unit tests that do not rely on
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- [ ] **Lint and test**: Run `make format`, `make lint` and `make test`
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Additional guidelines:
- Make sure optional dependencies are imported within a function.
- Please do not add dependencies to pyproject.toml files (even optional
ones) unless they are required for unit tests.
- Most PRs should not touch more than one package.
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If no one reviews your PR within a few days, please @-mention one of
baskaryan, efriis, eyurtsev, ccurme, vbarda, hwchase17.

libs/core/langchain_core/runnables/graph_png.py

@@ -174,7 +174,9 @@ class PngDrawer:
             graph: The graph to draw.
         """
         for start, end, data, cond in graph.edges:
-            self.add_edge(viz, start, end, str(data), cond)
+            self.add_edge(
+                viz, start, end, str(data) if data is not None else None, cond
+            )
 
     def update_styles(self, viz: Any, graph: Graph) -> None:
         """Update the styles of the entrypoint and END nodes.