refactor: 更新 avatar 组件验证逻辑

Author: ArgoZhang

src/BootstrapBlazor/Components/Upload/AvatarUpload.razor.cs

@@ -140,12 +140,12 @@ protected override void OnParametersSet()
 
     private string? GetValidStatus(UploadFile? item = null)
     {
-        if (item == null || IsDisabled || ValidateForm == null)
+        if (IsDisabled || ValidateForm == null)
         {
             return null;
         }
 
-        var state = item.IsValid;
+        var state = item?.IsValid ?? IsValid;
         return state.HasValue
             ? state.Value ? "is-valid" : "is-invalid"
             : null;