eda: full na column is not correct in report

[jinglinpeng]

Describe the bug The col1 should be na values, but it's missing value is zero in the Variables section.

To Reproduce

from dataprep.eda import create_report
import pandas as pd
import numpy as np
df = pd.DataFrame()
df["col1"] = [np.nan] * 10
create_report(df).show_browser()