refactor: 更新 Placeholder 逻辑

Author: ArgoZhang

src/BootstrapBlazor/Components/Select/MultiSelect.razor

@@ -9,7 +9,10 @@
 }
 <div @attributes="@AdditionalAttributes" class="@ClassString" id="@Id">
     <div class="@ToggleClassString" data-bs-toggle="@ToggleString" data-bs-placement="@PlacementString" data-bs-offset="@OffsetString" data-bs-auto-close="outside" data-bs-custom-class="@CustomClassString" tabindex="0">
-        <div class="@PlaceHolderClassString">@PlaceHolder</div>
+        @if(!CheckCanEdit())
+        {
+            <div class="@PlaceHolderClassString">@PlaceHolder</div>
+        }
         <div class="multi-select-items">
             @if (DisplayTemplate != null)
             {
@@ -37,7 +40,7 @@
             }
             @if (CheckCanEdit())
             {
-                <input type="text" class="multi-select-input" autocomplete="off" tabindex="0" data-bb-trigger-key="@EditSubmitKeyString" />
+                <input type="text" class="multi-select-input" autocomplete="off" tabindex="0" placeholder="@PlaceholderString" data-bb-trigger-key="@EditSubmitKeyString" />
             }
         </div>
         @if (!IsSingleLine)

src/BootstrapBlazor/Components/Select/MultiSelect.razor.cs

@@ -193,6 +193,8 @@ public partial class MultiSelect<TValue>
 
     private string? PreviousValue { get; set; }
 
+    private string? PlaceholderString => SelectedItems.Count == 0 ? PlaceHolder : null;
+
     /// <summary>
     /// OnParametersSet 方法
     /// </summary>