First-time data transform bugfix

[aimilefth]

Solves the following bug, when converting the data from raw to processed

root@coroni:/app# python3 privateer_ad/etl/transform.py
INFO:root:Initializing DataProcessor...
INFO:root:Loaded data from
/app/data/raw/amari_ue_data_merged_with_attack_number.csv
INFO:root:Setting up scalers from benign training data only...
WARNING:root:Found 580 NaT values in _time column.
INFO:root:Using 514430 benign samples out of 548101 total training
samples for scaling
Traceback (most recent call last):
  File "/app/privateer_ad/etl/transform.py", line 583, in <module>
    dp.prepare_datasets()
  File "/app/privateer_ad/etl/transform.py", line 115, in prepare_datasets
    self._save_scalers()
  File "/app/privateer_ad/etl/transform.py", line 203, in _save_scalers
    joblib.dump(self.scalers[feature_name], scaler_path)
                ~~~~~~~~~~~~^^^^^^^^^^^^^^
TypeError: 'NoneType' object is not subscriptable