Fix DataArray.to_netcdf type annotation (#3325)

It calls DataSet.to_netcdf, which returns Union[bytes, "Delayed", None].  So this should as well.

Author: Tony Tung

xarray/core/dataarray.py

@@ -2244,7 +2244,7 @@ def to_masked_array(self, copy: bool = True) -> np.ma.MaskedArray:
         isnull = pd.isnull(values)
         return np.ma.MaskedArray(data=values, mask=isnull, copy=copy)
 
-    def to_netcdf(self, *args, **kwargs) -> Optional["Delayed"]:
+    def to_netcdf(self, *args, **kwargs) -> Union[bytes, "Delayed", None]:
         """Write DataArray contents to a netCDF file.
 
         All parameters are passed directly to `xarray.Dataset.to_netcdf`.