Fixed: API error when sending payload without optional parameters

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

(cherry picked from commit 8d83b1d8d66ef24a25f312bdeac1e5625a630c00)

src/Readarr.Http/ClientSchema/SchemaBuilder.cs

@@ -46,7 +46,10 @@ namespace Readarr.Http.ClientSchema
             {
                 var field = fields.Find(f => f.Name == mapping.Field.Name);
 
-                mapping.SetterFunc(target, field.Value);
+                if (field != null)
+                {
+                    mapping.SetterFunc(target, field.Value);
+                }
             }
 
             return target;