set nine patch padding initially to an empty rectangle

Formerly, the padding state was set to null. Copying the padding
state fails when the null rectangle contents are referenced.

fixes http://b/2219785

graphics/java/android/graphics/drawable/NinePatchDrawable.java

@@ -81,7 +81,7 @@ public class NinePatchDrawable extends Drawable {
      */
     @Deprecated
     public NinePatchDrawable(NinePatch patch) {
-        this(new NinePatchState(patch, null), null);
+        this(new NinePatchState(patch, new Rect()), null);
     }
 
     /**
@@ -89,7 +89,7 @@ public class NinePatchDrawable extends Drawable {
      * based on the display metrics of the resources.
      */
     public NinePatchDrawable(Resources res, NinePatch patch) {
-        this(new NinePatchState(patch, null), res);
+        this(new NinePatchState(patch, new Rect()), res);
         mNinePatchState.mTargetDensity = mTargetDensity;
     }