Remove unnecessary null pointer checks

[Markus Elfring]

An extra null pointer check is not needed in functions like the following.

• BsdfProperty::buildBsdfPage
• Permutation

[Benedikt Bitterli]

Hi!

One of these is in embree, which is a third-party library that I would prefer not to change. I'm not too comfortable removing the null pointer check in the first example, as I faintly remember a reason for it being there.

Thanks!