fix #1123

- correctly handles @null in xml
 - added unit test

brut.apktool/apktool-lib/src/main/java/brut/androlib/res/data/value/ResValueFactory.java

@@ -34,7 +34,7 @@ public class ResValueFactory {
     public ResScalarValue factory(int type, int value, String rawValue) throws AndrolibException {
         switch (type) {
             case TypedValue.TYPE_NULL:
-                if (value == TypedValue.DATA_NULL_EMPTY) { // Special case $empty as explicitly defined empty value
+                if (value == TypedValue.DATA_NULL_UNDEFINED) { // Special case $empty as explicitly defined empty value
                     return new ResStringValue(null, value);
                 }
                 return new ResReferenceValue(mPackage, 0, null);

brut.apktool/apktool-lib/src/test/resources/brut/apktool/testapp/res/values-mcc001/styles.xml

@@ -9,4 +9,7 @@
         <item name="@string/issue_477">?android:dividerVertical</item>
     </style>
     <style name="Foo.Bar" parent=""/>
+    <style name="IssueStyle">
+        <item name="@string/issue_1123">@null</item>
+    </style>
 </resources>

brut.apktool/apktool-lib/src/test/resources/brut/apktool/testapp/res/values/strings.xml

@@ -3,4 +3,5 @@
     <string name="test1" />
     <string name="hello_world">Hello World</string>
     <string name="issue_477">divider</string>
+    <string name="issue_1123">divider</string>
 </resources>