ARROW-3735: [Python] Add test for calling cast() with None
ARROW-3735: [Python] Add test for calling cast() with None

Author: Korn, Uwe <Uwe.Korn@blue-yonder.com>

Closes #3862 from xhochy/ARROW-3735 and squashes the following commits:

0f91e5f59 <Korn, Uwe> ARROW-3735:  Add test for calling cast() with None

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