fix single pandas index non-dimension coord name

Author: benbovy

xarray/core/indexes.py

@@ -701,7 +701,7 @@ def create_variables(
             encoding = None
 
         data = PandasIndexingAdapter(self.index, dtype=self.coord_dtype)
-        var = IndexVariable(self.dim, data, attrs=attrs, encoding=encoding)
+        var = IndexVariable(self.dim, data, attrs=attrs, encoding=encoding, name=name)
         return {name: var}
 
     def to_pandas_index(self) -> pd.Index: