Verify if we correctly check for null values

[gijzelaerr]

This PR changes how we check for null values using a list comprehension:

https://github.com/MonetDBSolutions/MonetDBe-Python/pull/167/files#diff-50400c771b1134baeb7e4c99cb21a03fc2dcb0e18d76e63260d88ada8ce14156R40

we need to make sure this is the fastest way to do it, maybe the older numpy method was faster.