Enable nullable annotations for Volo.Abp.Json.Abstractions

framework/src/Volo.Abp.Json.Abstractions/Volo.Abp.Json.Abstractions.csproj

@@ -5,6 +5,8 @@
 
     <PropertyGroup>
         <TargetFrameworks>netstandard2.0;netstandard2.1;net7.0</TargetFrameworks>
+        <Nullable>enable</Nullable>
+        <WarningsAsErrors>Nullable</WarningsAsErrors>
         <PackageId>Volo.Abp.Json.Abstractions</PackageId>
         <AssetTargetFallback>$(AssetTargetFallback);portable-net45+win8+wp8+wpa81;</AssetTargetFallback>
         <GenerateAssemblyConfigurationAttribute>false</GenerateAssemblyConfigurationAttribute>

framework/src/Volo.Abp.Json.Abstractions/Volo/Abp/Json/AbpJsonOptions.cs

@@ -12,7 +12,7 @@ public class AbpJsonOptions
     /// <summary>
     /// Format of output json date, Null or empty string means default format.
     /// </summary>
-    public string OutputDateTimeFormat { get; set; }
+    public string? OutputDateTimeFormat { get; set; }
 
     public AbpJsonOptions()
     {