fix(api types): Add an unused field to empty API types.

Null structs (struct Foo;) cause the following error when trying to
deserialize an empty JSON object `{}` into them:

`JsonDecodeError("{}\n", Syntax(InvalidType(Map), 1, 1))` (also known as
`invalid type: map at line 1 column 1: {}`). The optional struct member
prevents this error.

gen/appengine1_beta4/src/lib.rs.in

@@ -371,7 +371,7 @@ impl Part for Library {}
 /// * [modules versions instances debug apps](struct.AppModuleVersionInstanceDebugCall.html) (request)
 /// 
 #[derive(Default, Clone, Debug, Serialize, Deserialize)]
-pub struct DebugInstanceRequest;
+pub struct DebugInstanceRequest { _never_set: Option<bool> }
 
 impl RequestValue for DebugInstanceRequest {}