Add missing @Nullable annotation on the keystrokes parameter of setFocusTraversalKeys. (#133)

I made the change in `Container` (where the behavior for `null` is
documented) and its subclass `JComponent` (where it's not). I didn't
search for other overrides of the method.

Author: cpovirk

src/java.desktop/share/classes/java/awt/Container.java

@@ -3134,7 +3134,7 @@ public void list(PrintWriter out, int indent) {
      * @since 1.4
      */
     public void setFocusTraversalKeys(int id,
-                                      Set<? extends AWTKeyStroke> keystrokes)
+                                      @Nullable Set<? extends AWTKeyStroke> keystrokes)
     {
         if (id < 0 || id >= KeyboardFocusManager.TRAVERSAL_KEY_LENGTH) {
             throw new IllegalArgumentException("invalid focus traversal key identifier");

src/java.desktop/share/classes/javax/swing/JComponent.java

@@ -4255,7 +4255,7 @@ void setUIProperty(String propertyName, Object value) {
      * @since 1.5
      */
     public void
-        setFocusTraversalKeys(int id, Set<? extends AWTKeyStroke> keystrokes)
+        setFocusTraversalKeys(int id, @Nullable Set<? extends AWTKeyStroke> keystrokes)
     {
         if (id == KeyboardFocusManager.FORWARD_TRAVERSAL_KEYS) {
             setFlag(FOCUS_TRAVERSAL_KEYS_FORWARD_SET,true);