Annotate the parameter of List.sort as @Nullable.

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](https://github.com/google/guava/blob/e82e2a2c07c68108f318958ee0355cc835c97743/guava-testlib/src/com/google/common/collect/testing/testers/SortedSetNavigationTester.java#L57)).
And while I prefer a world in which methods like `comparator()` never
return `null`, as we arranged for [in
`SortedMultiset`](https://guava.dev/SortedMultiset#comparator()), there
are apparently [downsides](google/guava#6187)
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.

src/java.base/share/classes/java/util/List.java

@@ -514,7 +514,7 @@ default void replaceAll(UnaryOperator<E> operator) {
  * @since 1.8
  */
  @SuppressWarnings({"unchecked", "rawtypes"})
- default void sort(Comparator<? super E> c) {
+ default void sort(@Nullable Comparator<? super E> c) {
  Object[] a = this.toArray();
  Arrays.sort(a, (Comparator) c);
  ListIterator<E> i = this.listIterator();