Missing values in chapter-18 dataset.

[FabianKP]

Hi Marco,
I really enjoy your book and I already learned a lot! I have a question regarding how you deal with the missing values in the power consumption dataset. Because there are missing values also in the columns other than "Sub_metering_3". One doesn't see them in the notebook since at the time where df.isna().sum() is executed, these columns are still filled with Strings. But when I run your notebook and execute df.isna().sum() AFTER doing the data conversion, I get the following:

I was unsure whether you are aware of this issue since you write at the end of chapter 18.2.1 that "We no longer have any missing data in our dataset".