Issue: 237 - Made Date Property Nullable

Updated c# from 7.3 to 8 for nullable reference type support

Author: vijayvarmaD

Src/Notion.Client/Models/PropertyValue/DatePropertyValue.cs

@@ -13,8 +13,8 @@ public class DatePropertyValue : PropertyValue
         /// <summary>
         /// Date
         /// </summary>
-        [JsonProperty("date")]
-        public Date Date { get; set; }
+        [JsonProperty("date", NullValueHandling = NullValueHandling.Include)]
+        public Date? Date { get; set; }
     }
 
     /// <summary>

Src/Notion.Client/Notion.Client.csproj

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <VersionPrefix>2.2.3-preview</VersionPrefix>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <LangVersion>7.3</LangVersion>
+    <LangVersion>8.0</LangVersion>
 
     <PackageId>Notion.Net</PackageId>
     <Authors>Vedant Koditkar</Authors>