Annotate the parameter in TextUtils#isEmpty() as @Nullable

Change-Id: Iaccf7f568952399d5eb48c15a5428317dd75052a

core/java/android/text/TextUtils.java

@@ -16,6 +16,7 @@
 
 package android.text;
 
+import android.annotation.Nullable;
 import android.content.res.Resources;
 import android.os.Parcel;
 import android.os.Parcelable;
@@ -457,7 +458,7 @@ public class TextUtils {
      * @param str the string to be examined
      * @return true if str is null or zero length
      */
-    public static boolean isEmpty(CharSequence str) {
+    public static boolean isEmpty(@Nullable CharSequence str) {
         if (str == null || str.length() == 0)
             return true;
         else