Fix mypy error by conditionally setting plot_node["chart-var-name"] only when option is present

- The original `**{"chart-var-name": ...}` likely existed because chart-var-name contains a hyphen, so it cannot be passed as a normal keyword argument.
- New behavior: create node first, then only set plot_node["chart-var-name"] when user provided that option.
- Downstream code uses node.get("chart-var-name", None), so behavior is effectively the same for runtime logic.

Author: joelostblom

sphinxext_altair/altairplot.py

@@ -212,8 +212,9 @@ def run(self) -> list[nodes.Element]:
             links=self.options.get("links", app.builder.config.altairplot_links),
             output=self.options.get("output", "plot"),
             strict="strict" in self.options,
-            **{"chart-var-name": self.options.get("chart-var-name", None)},
         )
+        if "chart-var-name" in self.options:
+            plot_node["chart-var-name"] = self.options["chart-var-name"]
         if "alt" in self.options:
             plot_node["alt"] = self.options["alt"]