Enable nullable in generated code to fix warning

Author: kristiker

ElementFactoryGenerator/ElementFactory.cs

@@ -31,6 +31,8 @@ private void Execute(SourceProductionContext context, (Compilation Left, Immutab
 
         elementFactory.Append(
             """"
+            #nullable enable
+
             public class ElementFactory : Datamodel.Codecs.IElementFactory
             {
                 public object? GetClass(string assembly, string nameSpace, string classname)
@@ -287,4 +289,4 @@ private class FactoryNamespace(string name)
         public string Name = name;
         public List<INamedTypeSymbol> Types = new();
     }
-}
+}