API: bool(pd.NA)
xref https://github.com/pandas-dev/pandas/pull/38102#discussion_r532446692

for `np.nan` we define `bool(np.nan)`
```
In [189]: 1 or np.nan                                                                                                                                                                     
Out[189]: 1

In [190]: np.nan or 1                                                                                                                                                                     
Out[190]: nan
```

however we don't do this for `pd.NA`
```
>>> pd.NA or 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/_libs/missing.pyx", line 360, in pandas._libs.missing.NAType.__bool__
    raise TypeError("boolean value of NA is ambiguous")
TypeError: boolean value of NA is ambiguous
```

I think this is pretty odd and not very useful.