UI Design - Allow a firm registration (SP/GP), for an user/account that only uses a Last Name

@yuisotozaki We have a high level mandate to complete this work during this sprint starting today, 19.1. Would much appreciate your help with this mini design ticket, at your convenience. While we expressed some opinions here, we count on you and the UX team to recommend which way to go.
- [ ] On the completing party component, the first name will become optional, so if the logged-in account has an empty first name, the validation does not fail.
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