Remove UINavigationBar shadowImage trying to send [UIImage new] when select setShadowImage:

"UINavigationBar": {
        "backgroundImage:default": "nav_bg",
        "shadowImage": "",
    }

When trying to set custom background image and remove the default one point height shadow image, I have to set a UIImage instance without content == [UIImage new], but in - (BOOL)convertValue:(id)value imageHandler:(void (^)(UIImage *))imageHandler codeHandler:(void (^)(NSString *))codeHandler; if the value is @"", [UIImage imageNamed:value] will return nil so the shadow image can not be removed.