Assigned @null default value to crop_image_menu_crop drawable

Undefined resource value isn't allowed on Gradle plugin 3.0-alpha2 and above

Author: adrien-aubel

cropper/src/main/res/values/ids.xml

@@ -1,4 +1,4 @@
 <?xml version="1.0" encoding="utf-8"?>
 <resources>
-    <item name="crop_image_menu_crop" type="drawable"/>
-</resources>
+    <item name="crop_image_menu_crop" type="drawable">@null</item>
+</resources>