Fixed: API error when sending payload without optional parameters

Co-authored-by: Qstick <qstick@gmail.com>

src/Sonarr.Http/ClientSchema/SchemaBuilder.cs

@@ -46,8 +46,11 @@ namespace Sonarr.Http.ClientSchema
                 var propertyType = mapping.PropertyType;
                 var field = fields.Find(f => f.Name == mapping.Field.Name);
 
+                if (field != null)
+                {
                     mapping.SetterFunc(target, field.Value);
                 }
+            }
 
             return target;