Add size check and adaptation to handle the changes in polars.hstack

[rpessi]

Refactor moving average calculation to handle empty DataFrame case.

[mmantyla]

Thanks. The old code might not work with the more recent polars versions. Also the old is not really in Polars spirit. Perhaps while we are at it, you could update the pull request like this and test if it fixes the issue? This approach should also be faster but I have not tested it.

return df.select([
        pl.col(col).rolling_mean(window_size).alias(f"moving_avg_{window_size}_{col}")
        for col in numeric_cols

[rpessi]

I updated with the suggested return statement, used my descretion to add the missing brackets.