How to read NULL and blank in the same dataframe

The data I have is :

col1   col2      col3
 a             null value

When I load it into a pandas dataframe with pd.read_csv, I get:

 col1   col2    col3
  a      NaN    NaN

I don't want the blank space to be read as NULL,

so I have added pd.read_csv('',na_filter=False) ,

but with this the NULL value is also getting skipped, and I get:

col1   col2    col3
 a             

The desired output is:

col1   col2    col3
 a              NaN

How to achieve this?