Fix the bug and make None as NaN value

[yixuy]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

This branch tends to fix the bug when the user input a country name as None, the library will return 'Niue' using clean_country from dataprep.clean

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

The test code as follow:

import pandas as pd
import numpy as np
from dataprep.clean import clean_country

df = pd.DataFrame({"id": [1,2,3,4,5], "country": ["United States","Kanada", "Fimland",np.nan,None]})
df = clean_country(
    df=df,
    column="country",
    input_format="auto",
    output_format="name",
    fuzzy_dist=2,
    strict=False,
    inplace=False,
    errors="coerce",
    report=True,
    progress=True
)
df

Snapshots:

Include snapshots for easier review.

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have already squashed the commits and make the commit message conform to the project standard.
• [X] I have already marked the commit with "BREAKING CHANGE" or "Fixes #" if needed.
• [X] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules

[codecov[bot]]

Codecov Report

Merging #814 (d7ab93e) into develop (1be5fce) will increase coverage by 0.02%. The diff coverage is 83.33%.

@@             Coverage Diff             @@
##           develop     #814      +/-   ##
===========================================
+ Coverage    55.11%   55.13%   +0.02%     
===========================================
  Files          293      293              
  Lines        18855    18870      +15     
===========================================
+ Hits         10391    10404      +13     
- Misses        8464     8466       +2     

Impacted Files	Coverage Δ
dataprep/clean/clean_lat_long.py	86.22% <ø> (ø)
dataprep/clean/clean_date_utils.py	76.84% <81.25%> (+0.25%)	:arrow_up:
dataprep/clean/clean_country.py	93.75% <100.00%> (ø)
dataprep/eda/correlation/compute/overview.py	99.24% <0.00%> (ø)
dataprep/eda/diff/render.py	91.33% <0.00%> (+0.36%)	:arrow_up:

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