nuageinit: Skip ssh keys if default user adduser fails

[emaste]

Summary

Commit d9fa6016011e ("nuageinit; Skip SSH keys and doas/sudo if adduser
fails") handled nil return from a failing adduser when adding secondary
users, but the same issue also existed in the default user handling.

Reported by:    llm-postcr
Supported by:   The FreeBSD Foundation