[Python] Refactor handling of null sentinels in python/numpy_to_arrow.cc

[asfimport]

See comments in ad30138

Reporter: Wes McKinney / @wesm
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [Python] NaN values silently casted to int64 when passing explicit schema for conversion in Table.from_pandas (relates to)

_{Note: This issue was originally created as ARROW-1899. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
I believe this was fixed in ARROW-2135.

[asfimport]

Wes McKinney / @wesm:
Thanks @pitrou – I resolved this