fix CacheManager GetLocalizedString Key.Value Null

Author: densen2014

src/BootstrapBlazor/Services/CacheManager.cs

@@ -242,7 +242,7 @@ private static JsonLocalizationOptions GetJsonLocalizationOption()
             var sections = Instance.GetOrCreate(key, entry => option.GetJsonStringFromAssembly(assembly, cultureName));
             var items = sections.FirstOrDefault(kv => typeName.Equals(kv.Key, StringComparison.OrdinalIgnoreCase))?
                 .GetChildren()
-                .SelectMany(kv => new[] { new LocalizedString(kv.Key, kv.Value!, false, typeName) });
+                .SelectMany(kv => new[] { new LocalizedString(kv.Key, kv.Value?? kv.Key, false, typeName) });
 #if NET8_0_OR_GREATER
             return items?.ToFrozenSet();
 #else