refs: Make ostree_repo_resolve_rev{,_ext}() use (nullable)
refs: Make ostree_repo_resolve_rev{,_ext}() use (nullable)

We have an `allow_noent` boolean that controls this, but
were missing the `(nullable)` annotation, so the Rust bindings
panic when the ref doesn't exist instead of being `Option<GString>`.

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree