Fix checking if value is None in _serialize.py

[morkovka1337]

This patch makes check more pythonic way and fixes an error of checking array values

[thijsmie]

Hi @morkovka1337,

According to the type this value can already be not be None (use Optional for that). This if statement here is purely to guard against zero-length-arrays (which evaluate to False when forced to boolean). The pre-PR state of this code was correct :)

[eboasson]

Trusting @thijsmie here, and so closing issue. But I also want to thank you @morkovka1337. Most grateful to everyone who puts effort into helping develop Cyclone!