String-to-boolean conversion is different from Pandas

[skirui-source]

Fixes: #7875

Previously: Pandas treats all non-empty strings as true values when it converts strings to booleans, whereas cuDF accepts only those that match with the true string (which is True by default).

This PR resolves the mismatch by introducing the str_to_boolean method, which filters a string column to check if len(StringColumn)> 0 and replaces NAN values with False to mimick Pandas behavior

Example:

>>> import pandas as pd
>>> import cudf

>>> gs = cudf.Series(["True", None, "", "True", "False", "False"])
>>> gs
0     True
1     <NA>
2         
3     True
4    False
5    False
dtype: object

>>> gs.astype(bool)
0     True
1    False
2    False
3     True
4     True
5     True
dtype: bool

>>> gs.to_pandas().astype(bool)
0     True
1    False
2    False
3     True
4     True
5     True
dtype: bool

[ttnghia]

FYI, please see https://nvidia.slack.com/archives/CDTANRCTT/p1616688610161600

[codecov]

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[skirui-source]

@gpucibot re-run tests

[skirui-source]

@gpucibot merge