fix: enhance null check for schema definitions in JsonSchema generation

Author: IvanMurzak

ReflectorNet.Tests/SchemaTests/SchemaTestBase.cs

@@ -153,7 +153,7 @@ protected void AssertResultNotRequired(JsonNode schema)
         }
 
         /// <summary>
-        /// Asserts that "result" IS in the required array (for non-nullable types
+        /// Asserts that "result" IS in the required array (for non-nullable types)
         /// </summary>
         protected void AssertResultRequired(JsonNode schema)
         {

ReflectorNet/src/Utils/Json/JsonSchema.cs

@@ -158,7 +158,8 @@ public JsonNode GetSchema(Reflector reflector, Type type, JsonObject? defines =
                         defines[defTypeId] = new JsonObject { [Type] = Object };
 
                         var def = GetSchema(reflector, defType, defines);
-                        defines[defTypeId] = def;
+                        if (def != null)
+                            defines[defTypeId] = def;
                     }
                 }
                 else