[Python] `tzinfo_to_string` should accept None

[asfimport]

Currently tzinfo_to_string errors with Not an instance of datetime.tzinfo when timezone is None. 

>>> tz = None
>>> import pyarrow
>>> pyarrow.lib.tzinfo_to_string(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pyarrow/types.pxi", line 2080, in pyarrow.lib.tzinfo_to_string
    return frombytes(GetResultValue(TzinfoToString(<PyObject*>tz)))
  File "pyarrow/error.pxi", line 143, in pyarrow.lib.pyarrow_internal_check_status
    return check_status(status)
  File "pyarrow/error.pxi", line 122, in pyarrow.lib.check_status
    raise ArrowTypeError(message)
pyarrow.lib.ArrowTypeError: Not an instance of datetime.tzinfo

tzinfo_to_string should accept None also. With this arrays() in python/pyarrow/tests/strategies.py in case of timestamp dtype should also accept None.

See

• https://github.com/apache/arrow/pull/12522#discussion_r822637011
• ARROW-15580: [Python] Make pytz an actual optional dependency of PyArrow #12522 (comment)

Reporter: Alenka Frim / @AlenkaF

_{Note: This issue was originally created as ARROW-16056. Please see the migration documentation for further details.}

[github-actions]

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