fix(steps): avoid bool-to-int cast and handle NULL

[jitingxu1]

This PR added the following

• use ifelse to avoid casting Boolean to Int
• Handle issues in bug(steps): OneHotEncode treats NaNs differently #54
• Added unit test for one hot encoding

>>> import ibis
>>> import ibisml as ml
>>> import pandas as pd
>>>
>>> t_train = ibis.memtable(
...         {
...             "time": [
...                 pd.Timestamp("2016-05-25 13:30:00.023"),
...                 pd.Timestamp("2016-05-25 13:30:00.023"),
...                 pd.Timestamp("2016-05-25 13:30:00.030"),
...                 pd.Timestamp("2016-05-25 13:30:00.041"),
...                 pd.Timestamp("2016-05-25 13:30:00.048"),
...                 pd.Timestamp("2016-05-25 13:30:00.049"),
...                 pd.Timestamp("2016-05-25 13:30:00.072"),
...                 pd.Timestamp("2016-05-25 13:30:00.075"),
...             ],
...             "ticker": ["GOOG", "MSFT", "MSFT", "MSFT", None, "AAPL", "GOOG", "MSFT"],
...         }
...     )
>>> t_test = ibis.memtable(
...         {
...             "time": [
...                 pd.Timestamp("2016-05-25 13:30:00.023"),
...                 pd.Timestamp("2016-05-25 13:30:00.038"),
...                 pd.Timestamp("2016-05-25 13:30:00.048"),
...                 pd.Timestamp("2016-05-25 13:30:00.049"),
...                 pd.Timestamp("2016-05-25 13:30:00.050"),
...                 pd.Timestamp("2016-05-25 13:30:00.051"),
...             ],
...             "ticker": ["MSFT", "MSFT", "GOOG", "GOOG", "AMZN", None],
...         }
...     )
>>> step = ml.OneHotEncode("ticker")
>>> step.fit_table(t_train, ml.core.Metadata())
>>> res = step.transform_table(t_test)
>>> res

outputs:

━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━━━━┳━━━━━━━━━━━━━┓
┃ time                    ┃ ticker_AAPL ┃ ticker_GOOG ┃ ticker_MSFT ┃ ticker_None ┃
┡━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━━━━╇━━━━━━━━━━━━━┩
│ timestamp               │ int8        │ int8        │ int8        │ int8        │
├─────────────────────────┼─────────────┼─────────────┼─────────────┼─────────────┤
│ 2016-05-25 13:30:00.023 │           0 │           0 │           1 │           0 │
│ 2016-05-25 13:30:00.038 │           0 │           0 │           1 │           0 │
│ 2016-05-25 13:30:00.048 │           0 │           1 │           0 │           0 │
│ 2016-05-25 13:30:00.049 │           0 │           1 │           0 │           0 │
│ 2016-05-25 13:30:00.050 │           0 │           0 │           0 │           0 │
│ 2016-05-25 13:30:00.051 │           0 │           0 │           0 │           1 │
└─────────────────────────┴─────────────┴─────────────┴─────────────┴─────────────┘

Resolves #54

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.32%. Comparing base (6bd2dcb) to head (98d109e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
+ Coverage   81.04%   83.32%   +2.27%     
==========================================
  Files          19       19              
  Lines        1361     1373      +12     
==========================================
+ Hits         1103     1144      +41     
+ Misses        258      229      -29     

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[deepyaman]

Minor feedback, most significantly on simplifying the test.