当 DataContext 为 null 时，绑定将使用 TargetNullValue 属性中指定的值.因此，尝试将 Visibility 绑定的 TargetNullValue 属性设置为 Collapsed 并且您甚至不需要转换器 :

When DataContext is null, binding will use value specified in TargetNullValue property. Therefore, try to set TargetNullValue property of Visibility binding to Collapsed and you won't even need converter :

.......
<Image 
    .......
    Visibility="{Binding userImage, TargetNullValue=Collapsed}" 
    />
.......