fix: init arg checks

Author: Joanna May

Chickensoft.Introspection.Generator/src/models/DeclaredType.cs

@@ -499,7 +499,8 @@ IEnumerable<DeclaredType> baseTypes
       .Where(prop => prop.HasSetter)
       .Select(
         (prop) =>
-          $"{prop.Name} = ({prop.GenericType.ClosedType})args[\"{prop.Name}\"]"
+          $"{prop.Name} = args.ContainsKey(\"{prop.Name}\") " +
+          $"? ({prop.GenericType.ClosedType})args[\"{prop.Name}\"] : default!"
       );
 
     writer.WriteCommaSeparatedList(