Annotate the parameter of List.sort as @Nullable.

[cpovirk]

This has always struck me as a weird feature, but people use it, and it
"works" in the sense of "does not cause NPE" (though it may cause
CCE...). I see both calls to sort(null) and calls like
sort(priorityQueue.comparator()) (which might be null;
example).
And while I prefer a world in which methods like comparator() never
return null, as we arranged for in
SortedMultiset, there
are apparently downsides
to using a Comparator that implements natural order rather than using
null.

Is any of that convincing? :) This is definitely a case in which I can
see how eisop would want to stay on its own path. My main motivation for
doing this now is that I hear that the current signature is causing us
trouble during Java->Kotlin transpilation. I can get more details.

[wmdietl]

This makes perfect sense. The javadoc for that parameter explicitly says that null is a valid value.
The default implementation in List calls Arrays.sort, which is already properly annotated:

jdk/src/java.base/share/classes/java/util/Arrays.java

Line 1438 in f9395fb

public static <T extends @Nullable Object> void sort(T[] a, @Nullable Comparator<? super T> c) {

I see no reason to reject this from EISOP and would appreciate a PR.

[cpovirk]

Whoa, I did not even realize the Arrays precedent. Thanks!