add net9

Author: pchalamet

src/FSharp.MongoDB.Bson/FSharp.MongoDB.Bson.fsproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.1</TargetFramework>
+    <TargetFrameworks>net9.0;netstandard2.1</TargetFrameworks>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <Nullable>enable</Nullable>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>

src/FSharp.MongoDB.Bson/Serialization/Serializers/FSharpMapSerializer.fs

@@ -21,7 +21,7 @@ open MongoDB.Bson.Serialization.Serializers
 /// <summary>
 /// Serializer for F# maps.
 /// </summary>
-type FSharpMapSerializer<'KeyType, 'ValueType when 'KeyType : comparison>() =
+type FSharpMapSerializer<'KeyType, 'ValueType when 'KeyType : comparison and 'KeyType: not null>() =
     inherit SerializerBase<Map<'KeyType, 'ValueType>>()
 
     let serializer = DictionaryInterfaceImplementerSerializer<Dictionary<'KeyType, 'ValueType>>()