Embind's subclass `implement` methods were generated as returning `Class
| null` after the changes to pointer types in #22184. This could be
considered a regression as the implement method would never return null.

Previously, we had special handling so constructors were marked as
nonnull so in the TS definitions we didn't add `| null`. I've
generalized this approach to work for all function bindings so they can
now use a `nonnull<ret_val>` policy too avoid the `| null`.