Dotnet codegen: fix id value serialization (#560)

* Read out id value correctly

Author: ZimmerA

schema_salad/dotnet_codegen.py

@@ -707,7 +707,10 @@ def declare_field(
             baseurl = "baseUrl"
         elif self.id_field_type.instance_type is not None:
             if self.id_field_type.instance_type.startswith("OneOf"):
-                baseurl = f"this.{self.safe_name(self.idfield)}.AsT1"
+                baseurl = (
+                    f"(this.{self.safe_name(self.idfield)}.Value is "
+                    f'None ? "" : {self.safe_name(self.idfield)}.Value)'
+                )
             else:
                 baseurl = f"this.{self.safe_name(self.idfield)}"