refactor: 使用空集合代替 new 提高性能

Author: ArgoZhang

src/BootstrapBlazor/Services/DefaultIconTheme.cs

@@ -22,7 +22,7 @@ public FrozenDictionary<ComponentIcons, string> GetIcons()
     {
         if (!_options.Value.Icons.TryGetValue(_options.Value.ThemeKey, out var icons))
         {
-            icons = new Dictionary<ComponentIcons, string>().ToFrozenDictionary();
+            icons = FrozenDictionary<ComponentIcons, string>.Empty;
         }
         return icons;
     }