Make sure `Value` of `FixedLocalizableString` not null.

Because the `Name` of `LocalizedString` can't be `null`.

framework/src/Volo.Abp.Localization.Abstractions/Volo/Abp/Localization/FixedLocalizableString.cs

@@ -8,6 +8,7 @@ public class FixedLocalizableString : ILocalizableString
 
     public FixedLocalizableString(string value)
     {
+        Check.NotNull(value, nameof(value));
         Value = value;
     }
 

framework/src/Volo.Abp.Localization/Volo/Abp/Localization/LocalizableStringSerializer.cs

@@ -35,7 +35,12 @@ public class LocalizableStringSerializer : ILocalizableStringSerializer, ITransi
 
     public virtual ILocalizableString Deserialize(string value)
     {
-        if (value.IsNullOrEmpty() ||
+        if (value == null)
+        {
+            throw new AbpException($"{nameof(value)} can not be null!");
+        }
+
+        if (value.IsNullOrWhiteSpace() ||
             value.Length < 3 ||
             value[1] != ':')
         {