fix(fact): null values comparison

[riezebosch]

Expanded the null checks, we're facing this because right now we just add a lot of facts and cleanup the null values afterwards. You could consider use a record for Fact instead. Not sure if you consider supporting null values in facts, but it is not currently guarded either.

fixes #92

[dgm9704]

Thank you for taking the time to provide this fix!
However I would ask for you to first make a bug detailing the problem,
with steps to reproduce it and other information you think is pertinent.
I will then evaluate the situation and figure out what will be the solution.
(I tend to quite opinionated about the functionality and structure of the library,
and also about the code.)

[riezebosch]

#92