Allow static json converters to write null values

CHANGELOG.md

@@ -63,7 +63,7 @@ Improvements
 ### MLEM.Data
 Improvements
 - **Moved extension methods into matching namespaces to avoid unexpected suggestions**
-- Allow static json converters to throw an exception if a key does not exist
+- Allow static json converters to write null values and to throw an exception if a key does not exist
 
 Removals
 - **Removed obsolete types DynamicEnumConverter, CopyExtensions, DynamicEnum, NetBufferSerializer, and NetExtensions**

MLEM.Data/Json/StaticJsonConverter.cs

@@ -58,8 +58,12 @@ namespace MLEM.Data.Json {
         /// <param name="value">The value.</param>
         /// <param name="serializer">The calling serializer.</param>
         public override void WriteJson(JsonWriter writer, T value, JsonSerializer serializer) {
+            if (value == null) {
+                writer.WriteNull();
+                return;
+            }
             if (!this.inverse.TryGetValue(value, out var key))
-                throw new InvalidOperationException($"Cannot write {value} that is not a registered entry");
+                throw new KeyNotFoundException($"Cannot write {value} that is not a registered entry");
             writer.WriteValue(key);
         }