add more null checks

[CasDragon]

People in Discord were showing an error of

[RespecWrath] Error while gathering library data
[RespecWrath] System.NullReferenceException: Object reference not set to an instance of an object
  at RespecWrath.Main.PatchLibrary () [0x001ba] in 

With some logging, apparently selection.GetType() was returning Null, so I assume somehow they had a Null in the selection somehow.

Adding a Null check before adding it appears to have fixed the error and allowed the user to go through the respec process successfully. So then I added a Null check to the second selection as well, just incase.

Not sure if this will have ramifications later on for the user though

[CasDragon]

ADDB tells me I should add logging to track down what is returning the null, and if it's a mod, to report it to the author. I'm not updating the PR for that, as I'm not sure if it's actually a mod or just cursed User nonsense, so I'll leave that decision up to you.

[CasDragon]

Kuru had some suggestions about the null checks, and forced me to open VS again. I have not tested these changes like I did with my prior commit, but .NotNull() seems like it should do exactly what the if not null checks were doing regardless so....

The List<BlueprintFeature> tempbackgroundlist = [DeityBackground.BackgroundSelect]; change is because Kuru hated the var :owlcat_laugh:

[CasDragon]

And now ADDB has suggested changes as well. This is my last commit, if someone else suggests a change, I'm closing the PR 😓