Don't pay attention to string block on @null

When someone referenced an empty string by putting in the special value
"@null" the string block was kept, but when filtering themes this wasn't
filtered because the framework thought it was actually a null string.

Bug: 3000940
Change-Id: I1e9a0479ef761b3e388b562ae728c5da0f4b5deb

core/jni/android_util_AssetManager.cpp

@@ -1111,6 +1111,7 @@ static jboolean android_content_AssetManager_applyStyle(JNIEnv* env, jobject cla
         if (value.dataType == Res_value::TYPE_REFERENCE && value.data == 0) {
             DEBUG_STYLES(LOGI("-> Setting to @null!"));
             value.dataType = Res_value::TYPE_NULL;
+            block = kXmlBlock;
         }
 
         DEBUG_STYLES(LOGI("Attribute 0x%08x: type=0x%x, data=0x%08x",