tm: Remove a check for NULL subtypes.
tm: Remove a check for NULL subtypes.

Callers should be using the default self_type, and never be creating a
type with 0 inner types.

jesserayadkins/lily,jesserayadkins/lily,FascinatedBox/lily,FascinatedBox/lily,FascinatedBox/lily,jesserayadkins/lily,FascinatedBox/lily