Filling in missing values
This is a controversial subject with no clear best answer and myriad options, some of which are model-specific. You can drop them, replace them with extreme values, interpolate, replace with median, impute with nearest neighbors, etc. In all cases besides dropping them altogether, you make assumptions and create data where no data exists.  I can't tell you the best approach, but I can advise you to try multiple approaches and hope the end results are robust to choice.