Describe the bug
the downloaded data for air dataset contains nan

To Reproduce
run the local models, it will return a NaN error for the series

Expected behavior
models run and returns correct SMAPE

System (please complete the following information):

• Python version: 3.10
• darts version 0.34.0

Additional context
can fix via creating a cell to fill missing values since this is for demo purposes

from darts.utils.missing_values import fill_missing_values

air_train = [fill_missing_values(ts, fill='auto') for ts in air_train]