[Core,Drivers] Improve use of `Ref.is_null/valid`

[AThousandShips]

trafficstars

Use is_null over !is_valid and vice versa.

Split into parts for easier review.

[fire]

I agree that using "positive" boolean math is better, but I don't know why we're doing this. Is there a bug or issue relating to this by the general godot engine community?

[AThousandShips]

This is simply for conformity and clarity of code

[AThousandShips]

Haven't gone back over these to ensure no new ones have cropped up but I tend to keep an eye on reviews so there shouldn't be anything missing

[Repiteo]

I'll operate under the assumption that it's accurate; future tweaks likely won't need to be sectioned off like this anyway

[Repiteo]

Thanks!