Annotate the parameter of List.sort as @Nullable.

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

@@ -1753,7 +1753,7 @@ private void replaceAllRange(UnaryOperator<E> operator, int i, int end) {
 
     @Override
     @SuppressWarnings("unchecked")
-    public void sort(Comparator<? super E> c) {
+    public void sort(@Nullable Comparator<? super E> c) {
         final int expectedModCount = modCount;
         Arrays.sort((E[]) elementData, 0, size, c);
         if (modCount != expectedModCount)

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

@@ -529,7 +529,7 @@ 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();

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

@@ -1413,7 +1413,7 @@ public synchronized void replaceAll(UnaryOperator<E> operator) {
 
     @SuppressWarnings("unchecked")
     @Override
-    public synchronized void sort(Comparator<? super E> c) {
+    public synchronized void sort(@Nullable Comparator<? super E> c) {
         final int expectedModCount = modCount;
         Arrays.sort((E[]) elementData, 0, elementCount, c);
         if (modCount != expectedModCount)

src/java.base/share/classes/java/util/concurrent/CopyOnWriteArrayList.java

@@ -897,7 +897,7 @@ void replaceAllRange(UnaryOperator<E> operator, int i, int end) {
         setArray(es);
     }
 
-    public void sort(Comparator<? super E> c) {
+    public void sort(@Nullable Comparator<? super E> c) {
         synchronized (lock) {
             sortRange(c, 0, getArray().length);
         }