fix(MultiSelect): keep last value when items is null (dotnetcore#6924)

* refactor: 优化性能

* fix: 支持首次渲染未给定 Items 情况

* refactor: 改用 Any 方法

Author: ArgoZhang

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

@@ -213,6 +213,11 @@ protected override void OnParametersSet()
                 var list = _currentValue.Split(',', StringSplitOptions.RemoveEmptyEntries);
                 SelectedItems.AddRange(Rows.Where(item => list.Any(i => i.Trim() == item.Value)));
             }
+
+            if (SelectedItems.Count == 0)
+            {
+                _lastSelectedValueString = string.Empty;
+            }
         }
     }