Minor code clean-up

[Thorium]

Minor code clean-up:

• x = null -> isNull x because: https://latkin.org/blog/2015/05/18/null-checking-considerations-in-f-its-harder-than-you-think/
• Faster niceName, but when this library is updated to F# 6 (to use ValueSome instead of Some with [<return:Struct>] that (and some other parts) can be made better).