Consistant managment of blank values

[sstruzik]

Issue Description

Depending on the data source and data storage, empty cell can have several different value.
Make sure that on load all of those value are converted to consistent blank value

Steps to Reproduce (Bugs only)

1.create exposure with blank values from csv, parquet or a dataframe.
2. check the blank values are converted to the same value (at the moment for string type csv return np.nan, parquet return None, and a dataframe return ''